Stan Stephens gives a masterclass in this dark art: from the bottom of the crankcases to the top of the head!
Stan Stephens with nine pages of tips!
I’ve been doing masterclasses in the various models I specialise in and wondered whether I should do one on tuning, but one reader wrote in to say: “Would Stan keep it simple and start from the basics!” What is basic for some readers is complicated for others! This reminds me of a few years back when I decided to learn to play a musical instrument. I figured I’d choose the bass as it would be easier than a ‘normal’ guitar (four strings not six, y’see?). So I booked some lessons with a bass guitar tutor. When we sat down for my first lesson, he said: ‘Where do you want to start?’ And I replied: ‘From the very beginning.’ He started talking about 12-bar blues but when he realised I didn’t even know how to hold the thing he said: “Ahhhh from the VERY beginning then!” So, with that in mind, I will cover the basics of what tools are needed and how to use them and the tuning will be a generalisation, not of any particular model. I will not be going into how a two-stroke works since I am assuming you have some degree of knowledge! I have to say that or this article would be longer still! And Bertie doesn’t pay me that much! There are two types of porting equipment either air or electric. I have always used electric as when I started tuning it was in a room over a shop and I couldn’t have a compressor hammering away and electric tools are so much smaller and easier to use in confined spaces. I get all my equipment from CC Speciality in America. You will need a lathe or access to one or know a small local engineers that can help with machining head and barrel faces. To measure port heights and widths you will need internal dividers and vernier calipers and a depth gauge. The basic porting tools
needed are for the different ports. If you think of all the tools that a dentist uses it will give you some idea. Which tool to use depends on the port that you are working on. For instance if you are raising the top of an exhaust port and the port is fairly long you will obviously need a cutter with a long shank. Whereas if you are working on the inlet port, which usually is short, you can use a shorter cutter, which is easier to control. To raise or widen the transfer ports you need right-angle cutting tools. I use a variety of cutters, a coarse one to shift the main part of the metal, a thin flat one to make the roof of the port flat, a small round one for getting around corners and a diamond impregnated stone for finishing off and variations of these for particular jobs. The engines we are dealing with in CMM are classic road bike engines so you are not looking at getting the utmost from the engine, just livening it up, so it’s best to be conservative with the porting: remember you can’t put metal back on. There are limits to which ports can be widened or raised before giving problems, I will deal with these limits later. I will give any measurements of port heights in mm from the top face of the barrel.
So, we will start at the bottom and work our way up, so first of all we will have a look at the crankcases and see what improvements that can be made. There is not any worthwhile work that can be done in the lower part of the cases; the most important part is where the barrels meet the cases: they never seem to match each other! I have used a YPVS 350 crankcase as an example but the theory applies to all two-stroke engines. Not only is it important that the barrel matches the cases but also that the gasket matches them both. The area we are talking about is where the gases exit the crankcase and go up the barrel to the transfer ports, this area is not the transfer port itself, we will call this the catchment area. The gases obviously have a very short time to travel up from the catchment area to the transfer port and out into the combustion chamber, therefore any help we can give by removing obstructions, the better. On the YPVS the catchment area in the base of the barrel is considerably larger than in the cases, also the base gasket matches the cases not the barrel. First of all with a sharp blade cut the gaskets to match the base of the barrels, next with a felt-tip pen mark the top of the cases and place the matched gaskets on the cases and with a scriber mark around the gasket. This will give you a line to work to, don’t just match the edge of the cases to the line, try to replicate the shape and the flow of the original. The more accurate you are the better the match and the flow will be. Obviously be careful not to damage the gasket face on the cases. There is a lot of metal to remove from the crankcases to make them match the barrels. On other makes or models it may be the other way around and the metal will have to be removed from the barrels instead but the principles are the same. The other important part of the catchment area is the cylinder liner and the bridges between the bottoms of the transfer ports. Any flowing work in this area will be beneficial. The liner itself can be cut and raised to enlarge the catchment area also. On most Yamahas you can raise the cutaways in the liner by up to 8mm but don’t do it on 500 or 750 Kawasaki triples because they have a habit of cracking there anyway! On later engines with crankcase reeds
(e.g. DT125R, TZR250, KR1S and RGV) flowing the crankcases is even more beneficial. I am sure if the engine’s designer ever saw what the production line did to the cases he would have a fit! If you remove the reed block and look down the inlet port on one of these engines you will see that the spigots on the bottom of the barrels protrude into the ports. On the DT125R there is 12mm to be removed from the spigots and about 6mm on the TZR250 and KR1-S. The main reason that my production race KR1S were so fast in the late 1980s was the amount of work I did on the inlets in the crankcases. There are huge pieces of casting masking the reed valves and there are two crankcase bolts which have needlessly large castings around them in each inlet. With all the obstructions removed and the inlets flowed around to the transfer catchment areas I used to gain up to 8bhp! All the work to the crankcases we have covered here is free power, I am sure it was in the original design of the engines. It can all be carried out with the simplest of equipment and there is not a lot of chance that you can do any damage.
Next we move on to the inlet port. There are three main types of inlets, piston controlled, reed-valve and disc-valve, I will just be covering the first two here. The earlier two-strokes (i.e. pre Yamaha RD series) had piston-controlled inlets. This means that the inlet port is opened and closed by the piston as it goes up and down the cylinder bore. At first you would think that the earlier the inlet opens the more fuel/air would be drawn into the engine but the problem with a piston-controlled inlet is that with an earlier opening inlet port as the piston goes up the bore it means that the inlet closes later as the piston comes back down the bore. The limiting factor with this type of inlet is that if the port closes too late it will blow back through the carb and also make it hard or impossible to start. My advice would be if you want to experiment with inlet port timing do it by shortening the inlet side of the piston, not by lowering the bottom of the port, then if you have gone too far it is easy to replace the piston. The inlet port should be smooth but not polished, with no obstructions. Any inlet stubs or gaskets should be matched to the port. As we have seen, the limit of the bottom of the inlet port is the gases blowing back out. The limit of the top of the port is that when the piston is at bottom dead centre the lower piston ring must not show in the port, otherwise the ring will snag and break. I like to allow the top of the port to be 5mm lower than where the ring comes down to; this allows there to be a large chamfer at the top of the port. The chamfer is for when you fit the barrel to stop the ring snagging. There needs to be a chamfer at the bottom of the inlet to stop the bottom of the piston catching. Kawasaki’s way around the problem is by using a ‘larynx’ at the top of the port to support the ring at BDC. Other engines – like the YPVS – use a bridged inlet. We have covered the limits of the top and bottom of the inlet port. To obtain more
area, if you are using larger carbs, you will need to widen the port. I widen it to a maximum of 60% of the cylinder bore diameter, unless the width of the bottom of the piston is less than that. Moving on to the reed-valve inlet, the purpose of the reed-valve was to cure the failings of the piston-controlled inlet of blow-back. It was thought at first that the extra obstructions of the reed assembly would limit power but this proved groundless. The only problem was that on the earlier reed engines the Japanese were far too conservative with reed block size. When we used the RD400 and later the LCS in production racing we had to use the original reed blocks and I spent hundreds of hours on the dyno testing different ideas of flowing the reed blocks and different reed materials and thickness. The best results I achieved were by removing the bridges in the blocks and the divider at the end and using longer reeds which seated against each other at their tips. The thinking there was that at high revs when the reeds don’t have time to open and close there was absolutely no obstruction in the reed block. Obviously that would not be sensible for the road but would be an idea for someone preparing an engine for sprinting or hill-climbs. When not being used for production racing it was easier to use larger reed blocks from other models, e.g. TZ750 reed blocks. To see how the Japanese corrected the problems of too small reed blocks, the 350 YPVS came fitted with reed blocks the same size as the TZ750! The other obstruction to the inlet is the piston. For racing we removed the back of the piston completely but that would be too radical for the road. However, enlarging the holes in the piston to the width of the port will help. Don’t cut away the back of the pistons in a 350YPVS; the bridged inlet is too wide and it needs the piston to bear against the bridge. You can see that inlet port on the tuned reed-valve engine is open for 360 degrees. The limit of the top of the port is the same as the piston controlled inlet and I have found there is very little to be gained from lowering the port.
Now on to the exhaust port. Up until now most of the work has been to improve the efficiency of the engine without really altering its characteristics. The exhaust port is where all that changes. Naturally as with most things in life you don’t get something for nothing. By altering the exhaust port height and to a degree the width, you will raise the peak power (more bhp at higher revs), the trade-off being that you will lose bottom-end power. Altering the exhaust port width will give similar results without losing as much bottom-end power. The downside here is that you can’t widen the ex port too far or the piston rings will suffer as they try to
bend into the port. When raising the ex port to the required height for maximum power the two-stroke tuner would also raise the transfer ports to retain or improve the mid-range power but I am going to assume here that you don’t possess the equipment to raise the transfers. Bearing that in mind, it is best to be a little conservative in raising the ex port height. On older classic two-strokes the ex ports are fairly low but the engines are likely to become peakier sooner. With later reed-valve engines you can be more daring with height before you start making it too peaky and with engines with powervalves you can throw caution to the winds (almost!). There are a few parameters to keep in mind as the absolute maximum amounts you can modify the port by. Never exceed 70% of the bore size when widening the port and that is when using steel piston rings. With the old cast-iron rings the figure is much lower, in fact I wouldn’t widen the port at all with cast rings. Come to think of it, I wouldn’t use cast rings at all! Never exceed 50% of the stroke when raising the port, 70% width and 50% height is what you would expect in a top-end power race engine. The shape of the exhaust port is very important; it is the shape which persuades the rings to stay in their grooves. If the port is too square at its top edge, the rings will try to bulge into the port so the port has to have a curved top edge, but the more curved it is the lower the top-end power is. The wider the port is, the more curved the top of the port must be. Also the port must have a chamfer to help ring life and to stop them snagging in the port. Once again the larger the chamfer the easier on the rings but there is a loss in power with wider chamfers, so as you can see everything is a compromise. With a classic engine, especially for street use, I would recommend you keep basically to the existing shape, The shape must be symmetrical, i.e the curve of the top edge and the radius of where the top of the port meets the sides of the port must be the same on both sides. If the shape of the port is lop-sided or the radius is different from one side to the other the piston rings will be pushed around one way in the grooves more than the other. This is the main cause of ring pegs coming adrift. If you compare the porting on a trials engine to porting of a high performance engine you will see that the trials engine has a low, narrow ex port with a very curved top to the port. This produces its best power at very low revs. Coupled to a low compression, the trials engine has soft tractable power with a smooth spread of power but with little top-end performance. The ex port of the tuned sports engine has a wide, high exhaust port with a much flatter top to the port, this engine produces
its peak power much further up the rev-range: more revs, more power and more speed! Something else to consider when raising the ex port is that as you raise the port you are lowering the compression. On a two-stroke the compression does not start until the ex port closes, so the higher the port, the lower the compression. You must always work out how much you have raised the port and how much it has lowered the compression and machine the head to raise the compression to compensate. So far we have covered the shape of the port in the cylinder wall. With the port now higher and wider there will be a ‘neck’ in the shape of the port as it goes through the barrel towards the exhaust pipe. As much as is possible there must not be any narrowing of the port from where it leaves the cylinder to where it meets the exhaust pipe. If the engine has a flange or stub fitting for the pipe make sure there is no step or restriction, including the gasket. If the engine has powervalves, flow them to the new shape of the port. Once again make sure there is no restriction and when finished ensure that the powervalve cables are adjusted so that the powervalve fully opens the port. With the other ports the theory is that the ports should not be polished because the slightly rougher finish helps the fuel and air to mix as it goes through the engine, unlike the exhaust port where the fuel mix has done its job and needs to exit the cylinder asap; so polish the exhaust port as much as you like!
There are many articles out there concerning port timing; all of them assume you are tuning the engine for maximum power with race exhausts and large carbs. They never take into account bikes that don’t have the optimum exhausts, carbs and ignition and run an airbox. If you have tuned the engine to race spec, there is a specific height and area that the transfer ports should be when matched to an exhaust port that is at its max height and width, but we aren’t talking about fully developed race engines here. Basically, as the exhaust port is raised you lose some mid-range power. To fill in that mid-range is the job of the transfers; the higher the exhaust port relatively the higher the transfers must be, but on your classic engine you have only raised the exhaust port a conservative amount. On most classic engines the matching of the castings to the liners is very poor, especially in the transfer windows. If you have a right-angle drive porting tool my advice would be to make a really good job of matching the barrel casting with the liner and ensure all the transfers are exactly the same height. Usually that will have raised the transfers as well as making them more efficient. When all the porting work has been done chamfer the ports. Especially make sure there is a generous chamfer top and bottom of the exhaust port.
Heads and compression ratios
With all the porting to the barrels finished, it is time to turn our attention to the heads. The first steps are to work out how much you are going to machine off the head to raise the compression and what squish clearance to have. Unless you are going to just skim 0.5mm off the head to slightly raise the compression you are going to have to work it out using that horrible word from school: maths! I am going to start with compression ratios: on most classic two-stroke road bikes there is a power increase to be had by increasing the compression. There is always an optimum compression above
which the increase in heat negates the proposed increase in power. As we are not dealing with race engines here, it is wise to be conservative. Small increases in compression will be beneficial throughout the rev-range; larger increases in compression will benefit performance at lower revs but be detrimental to power at higher revs. For example, a motocross engine would run a higher compression than a road race engine if they were both running on the same fuel. On a classic two-stroke I would not recommend over a 7-1 compression ratio and you would need to use the best unleaded fuel with the highest octane rating. The ‘compression ratio’ is the ratio between the volume of the cylinder and volume of the combustion chamber. To calculate the compression ratio you need to measure both these volumes. At school I must have been very selective about what I remembered: how to measure volumes of cylinders was obviously deemed important to remember! Pi R squared x height! I always take Pi to be 3.142, R is the radius, the two means squared (which means times itself). In simple terms, for example on a 350LC with a bore of 64mm, the radius is half the bore, half 64 is 32. The stroke (or height) is 54mm. So our little sum is 3.142 x 32 x 32 x 54 = 173.7cc, this is the cylinder volume but if the cylinders have been rebored to (say 1mm oversize) that will alter the sum to 3.142 x 32.5 x 32.5 x 54 = 179.2. We now have to measure the other part of the ratio, the combustion chamber volume: this can’t be calculated, it has to be measured. To do so you will need a burette or cc measuring tube. Ideally the engine will be on the bench, not in the bike. Remove the cylinder head, clean-up the head and barrel faces and remove the spark plug. Position the piston at top dead centre (TDC) and smear some grease around the edges of the piston to seal it and wipe off the excess. Use a new head gasket, fit the head and torque it down. Position the engine so that the cylinders are upright and check that the piston is still at TDC. Fill your burette with paraffin to the zero mark and insert it into the plug-hole, release the paraffin until it comes level with the top of the plug-hole and read off the burette how much paraffin it has taken. If for example it shows 18cc, deduct 2cc for the plug-hole, which gives 16cc which is the volume of the combustion chamber. To work out the compression ratio, take the volume of the cylinder, ADD the volume of the combustion chamber and DIVIDE by the volume of the combustion chamber. Which is: 173.7 + 16 divide by 16 = 11.85-1 CR. If we were talking about four-strokes that would be the compression ratio, but with a two-stroke the compression does not start until the piston closes the exhaust port. The correct way to calculate the compression ratio on a two-stroke would be to measure from the top of the exhaust port to the top of the barrel, then when working out the volume of the cylinder instead of the stroke of 54mm in the equation, you would insert the port height, but for the purpose we are intending it is fair to generalise that the exhaust port is 50% of the way up the stroke (i.e. 27mm from the top of the 54mm stroke). In this way the
“Two-stroke tuning is an art – but one that can be learned. It’s often about getting the best out of the machine as the original designer intended, before the needs of mass-manufacture came into play.”
compression ratio we arrived at earlier would be halved, so instead of about 12-1 it would be 6-1. This on a two-stroke is called the ‘corrected compression ratio.‘
Not all classic two-strokes have squish heads: the GT750 doesn’t. The squish area of a cylinder head is the area around the outside of the domed combustion chamber. The purpose is to squish (squeeze) the petrol/air mixture into the centre of the combustion chamber where the spark-plug is. Without a squish head it would be very difficult to get a reasonable compression without a flat inefficient-shaped combustion chamber. The squish clearance is the distance between the piston at TDC and the squish area of the head. The tighter the clearance the more efficient the squish. The idea is not to have any mixture trapped in the squish area, it all needs to be squished into the combustion area. Any mixture trapped in the squish area is wasted, if 10% is trapped that is 10% power wasted. Any mixture trapped will cause detonation or pre-ignition, known as pinking, which will cause damage and seizures. The reason for having any clearance is to allow for any expansion or stretching of parts. On a classic engine you need at least 1mm of clearance between piston and head to allow for any stretching of heavy con-rods and pistons. Ideally the angle of the squish band should be the same as the angle of the dome of the piston top. The squish area should be 50% of the area of the combustion chamber, here comes Pi again! So 50% of a 64mm diameter combustion chamber would be 3.142 x 32 x 32 divided by 2. Measure the squish clearance before stripping the engine. You will need
some 1.5mm or 2mm multi-core solder. Remove the spark-plug and bend a length of solder into an ‘L’ shape and put it through the plug-hole so that the end of the ‘L’ touches the edge of the cylinder. With a spanner on the fly-wheel nut, turn the engine over backwards and forwards over TDC a few times to compress the solder. Remove the solder and measure with a vernier. This will tell you the squish clearance. You will then be able to work out how much to skim off the head or barrel. Squish bands and compression ratios are interlinked because if you alter one you will need to alter the other. From the above calculations you will be able to work out how much to take off the head. Next we move on to the lathe to do the machining.
To machine single cylinder heads, I have a mandrel fixed to an old chuck with a thread which screws into the head’s plughole. First of all I machine the face of the plughole on the mill to make sure the head runs true when screwed onto the mandrel. To machine twin cylinder heads I have a face-plate fixed to a chuck for the lathe. The face-plate is drilled and tapped for all the many different heads that I do. Again I machine the faces of the plugholes but this time it is important to make sure that both plugholes are exactly the same depth. I bolt the head to the face-plate using two domed spacers inside the combustion chambers with long bolts going through the plugholes and through two spacers exactly the same length and into the threaded holes in the face-plate. The head should then run true ready to skim the required amount off. With the head-face machined to raise the compression it’s time to machine the squish bands. I remove the face-plate from the lathe and fit the mandrel that I use for the single cylinder heads. With a twin cylinder head I screw the plug thread of one combustion chamber onto the mandrel. I set the tool post cutter to the required angle. Not all squish bands are the same angle (most Yams are 14˚ while an RG500 is 21˚). With the lathe on a slow speed I machine the squish band, I then machine the other side the same. So, there you go! Now you know a little about two-stroke tuning. Me? Well, since learning to play the bass guitar I’m now in a band called Two-stroke Smoke and we play some of the local pubs. See? Never too old to learn something new – so get tuning!
A tweaked and tuned Elsie is a hoot out on the road.
Wanna port a Powervalve?
Here we are matching the cases and roughing out with a coarse cutter.
You’ll need these porting tools.
Here’s the gasket cut to the base of the barrel.
Finally, the finished cases!
Here we are flowing the crankcase reed cases.
Yet more tools for the job!
Wanna tease more from your TZR?
And now flowing the bottoms of the barrels.
Checking for highs or lows.
It’s time for matching the exhaust port to the gasket and flange.
Et voila! The finished exhaust port.
Finishing the port all off.
And now checking the diameter.
Here’s a tuned exhaust port window.
Wanna keep your Kettle on the boil?
In comparison, here’s a standard and tuned inlet.
And now race tuned and standard pistons.
Here are both tuned and standard reed-blocks.
This is the Kawasaki ‘larynx’ inlet.
Here the tops of the transfers are all the same height, area and angles.
LEFT: Wanna Kwik Kwak?
LEFT: Measuring head volume with a burette.
Here we are machining the squish band.
Wanna kwikken your KR-1S?
A tuned head and a standard head.
The head is bolted to a face plate to skim head face.