SUZUKI RG400-500
A Stan Stephens masterclass!
HIf you want to know how to strip, tune and rebuild an RG400 or 500 (and big bore the bugger) there’s only one man to turn to…
ere’s the latest in my series of masterclasses on particular legendary two-stroke motors. This month it’s the Suzuki RG400/500. Now, over the last eight years I’ve written a fair few features about these motors, on the strip and rebuild as well as on tuning, big-boring and one on the disc-valves used by the RG family of engines: I’ve pulled these stories together and updated them where and when necessary to give you all the cumulative knowledge of my many years of working on the Suzuki square-four two-stroke. This month is part one, where we deal with stripping the motor and look at big-bore kits for both the RG400 and RG500 as well as tuning the motor. Next issue will be the all-important rebuild. So, let’s get stripping, shall we? When stripping an RG500/400 engine you will need a long-series 6mm Allen key socket to reach the barrel retaining bolts which are down inside the barrels. To remove the ignition flywheel you must use a proper
puller. It’s a fairly universal puller which fits most Yamaha two-strokes as well. Never use a three-legged puller or similar! The only other handy tool needed you can easily make yourself: on the back left and front right of the crankcases there are two places to fit extraction bolts. Use two 8mm bolts and nuts, grind or file the sides of the nuts to fit in the slots in the crankcases. Screw the bolts into the nuts and as you tighten the bolts they will jack the cases apart! Remember when stripping an engine it is not just a case of taking it apart, it is the time when you carry out your detective work. I always say this, but it’s true! Always inspect everything and make notes as you strip it. If you are not familiar with the engine, in this day and age take photos with your phone. Have some handy pots or small boxes to put the stripped parts into. It’s a good idea to put all the top-end parts in one pot and all the left-hand side parts in another and so on. With the engine on the bench I would recommend that before removing the heads you carry out a check which could save you hours of inconvenience later. The problem you are looking to prevent is that the threads of the head studs in the barrels are notorious for stripping. It is frustrating when you have rebuilt the engine and you are tightening the head nuts and some of them won’t torque down because the threads are stripping. It would then mean removing the barrels again to drill and Helicoil the stripped stud-holes. So before removing the heads loosen each head nut a couple of turns and torque it back down to 18lb-ft to make sure the threads are okay. It is far better to find out now rather than later. After checking the threads remove the cylinder heads. Inspect the heads for signs of the head gaskets blowing – check the combustion chambers for any damage – if there is any, keep an eye open for what caused it as you strip the rest of the engine. The damage may have been caused by broken piston rings or bits of big-end or they may have been from a previous blow-up that has been repaired. From standard there should be a step in the head where the combustion chamber meets the head face. If there isn’t, it means the head has been skimmed. Check that the diameter of the combustion chamber is correct, it should be 57mm: if it is less than 56.5mm the squish band area will need re-machining to 57mm. The RG400 is 50mm up to 51mm. To remove the cylinder barrels you will need the 6mm long series Allen key socket. There are four 8mm threaded Allen bolts holding down each barrel, make sure the hex heads are clear of debris and that the Allen socket is firmly inserted because
if you strip the Allen socket heads you will have a real problem! Remove the barrels and check the bores for wear and any damage. Check the exhaust port windows to see if the rings have been snagging and wearing the port square. Check the cylinder bore diameters to make sure they are all the same in case someone has just had one cylinder bored. The standard bore is 56mm the over-sizes are 56.5mm and 57mm. If your bores are already 57mm that is the last normal re-bore size and you will need a 560cc conversion with 59mm pistons (which I will cover later!) I have never seen an over-size RG400 piston kit, (there may be some) so if your bore is damaged you will need the 470cc conversion and I will cover that later as well. The RG500/400 suffers from small-end wear so always check the small-end eyes of the con-rods for pitting. I always replace the small-ends and gudgeon pins. Remove the generator cover and with the correct puller remove the ignition flywheel and the generator back-plate. Next remove the disc-valve outer covers and check for further problems. The RG is notorious for fuel leaking through the fuel
tap and past the carb needle valves. The outer disc-valve covers have a fibre face and this bubbles-up and distorts when soaked in petrol for long periods. The fibre facing is very important: the RG does not have outer crank-seals: the seal is the disc-valve pressing against the fibre facing as the disc rotates. When removing the disc-valve inner covers don’t be tempted to lever them off, you will damage them. Remove the counter-sunk screws from all four inner covers, then when all the crankcase bolts have been removed including the two 6mm screws (8mm headed) inside the crankcases, using your special extractor bolts jack the cases apart and the inner covers will come off at the same time with no levering. I always replace all the crank-seals, main bearings and big-end bearings as a matter of course, even if they look okay, you cannot check every ball and every roller! If you are only stripping the engine you do not need to go any further. You do not need to strip the clutch side or the gearbox. If you are stripping the gearbox as well, drain the oil and inspect it for any bits of gears or debris. Remove the sump and check in there also. Remove the gearbox sprocket and sleeve. Remove the oil-pump. If there is any damage on the gear-change shaft from the chain hitting it, file off any high spots because otherwise it will damage the cases as it is removed. Remove the right-hand clutch casing. Remove the clutch springs and plates and using a clutch holding tool remove the clutch tab washer and nut. Remove the gear-change shaft. Remove the circle of 6mm (10mm headed) nuts around the gear-box casing, the RG gearbox is of the cassette type and the complete assembly will now slide out. Thoroughly inspect all the gears and their locating dogs. Check that the gearbox is of the later type. You don’t need to remove the jack-shaft but if you do want to, remove the right-hand primary gear and bearing and the centre clamp and fiddle the shaft out. It is a bit of a fiddle but it is easier than me trying to explain it!
Big-bore kits
Right, now let’s get onto the Rg560cc and Rg470cc big-bore kits. With the availability of parts for the RG500 drying up and the scarcity of second-hand parts for them, it is up to people like myself in the trade to conjure up ways to keep them running in good condition. The RG500 only ever had two re-bore sizes – 0.5mm over-size and 1mm over-size. More than 20 years ago I came up with a big-bore conversion to salvage worn-out or damaged 1mm o/s barrels. At that time the RG125 Gamma was Suzuki’s sports 125cc bike but I found that in the Far East there was an RG 150cc Gamma. I arranged for some piston kits to be imported. The RG125 only had a 54mm bore and the RG500 had a 56mm bore but the RG150 had a 59mm bore which was ideal! They were also available in 0.5 and 1mm. I had already checked there was enough thickness in the liner. Today in 2017 I still do the conversion the same as I did originally and I haven’t had any failures so I must have got it about right! The RG150 pistons are not a direct replacement, they need machining and modifying. Both RG125 and RG500 have 2mm thick shims either side of the small-ends, the RG150 doesn’t.
This means 2mm has to be machined off the inside of each of the gudgeon pin bosses in the pistons. The piston skirts are also 6mm too long and have to be shortened to 60mm long in the lathe. The bottoms of the skirts are not shaped and at bottom dead centre the pistons hit the fly wheels so a chamfer has to be machined on the bottom corners of the pistons, I just do it by hand on the linisher. To bore from 56mm to 59mm takes six cuts at 0.5mm at a time that’s 24 bores altogether for the four barrels! The transfer ports come into the cylinders at an angle; as the barrel is bored the roof of the ports are lower and they have to be raised to the correct height. This is true with all re-bores but especially with a big-bore. The two main transfers are okay but the other five transfers and boost port need a lot of work doing to them: the rear boost port needs raising almost 10mm and the other ones about 5mm. There are no head gaskets available for the 570cc big-bore. I machine the squish bands in the heads to the diameter of the gasket, 57mm and use RG500 head gaskets. The crowns of the RG150 pistons are 0.8mm lower than the RG500 ones so the pistons miss the head gaskets. This may not be ideal but with the head being the same diameter as the gasket, the gasket is not sticking into the combustion chamber on its own. I know of a firm who copy my conversion but machine out the head gasket to 59mm which would be the correct thing to do but they have a lot of head gasket trouble and I don’t so I will just keep doing it my way! The added advantage of the piston being 0.8mm lower is that it brings the compression back to normal. The RG400 is more plentiful but only has the performance of a standard 350F2 YPVS Yamaha. The cylinders cannot be bored out to take RG500 pistons: the liners are not thick enough. The crankcases can be bored out to take RG500 barrels, that is if you could find RG500 barrels! My 470cc conversion uses RG125 Gamma pistons. The RG400 is 50mm bore and the RG125 is 54mm. The pistons are the right length and the bottoms of the skirts are chamfered to miss the flywheels, the width across the gudgeon pin bosses is right and the ring pegs are in the right place. There are 0.5mm and 1mm over-size pistons available. To bore the barrels from 50mm to 54mm is eight bores each barrel, a total of 32 bores! When bored, the spigots at the bottoms of the liners are very spindly and I shorten them by 7mm on the lathe. I use RG500 head gaskets and machine the squish bands in the heads out to 54mm. The RG400 has a low-compression and although the capacity is up, by machining the squish bands and using the RG500 gaskets the compression is okay. As on the 570 the ports on the 470 need raising after the boring. The RG400 only has five transfers instead of the RG500’S seven transfers. With some tuning as well the 470cc conversion produces more power than a standard 500. If you have damaged cylinder heads on your RG500 and can get hold of a couple of RG400 heads they can be machined out to the same dimensions. First I machine the plug-holes to the same depth, because I mount them on a faceplate and locate them on the plug-holes. I then skim the face of the heads. The RG400 combustion chambers are 50mm diameter and the RG500 is 56mm. I machine the squish bands out to 56mm diameter at 25º. I have already measured the volume in the RG500 heads: it’s 17cc to the top of the plug-hole measured on a face-plate. With the RG400 heads squishbands already machined I measure the volumes and they are only 15ccs so
there is 2cc to be machined out of the combustion chambers to bring them to the same spec as the RG500. I use a specially ground and shaped lathe tool to machine out the combustion chambers. No doubt if I had a CNC lathe I could machine them all the same, but I don’t so I have to keep taking them off and on the lathe and measuring the volume, very time consuming but very necessary.
Tuning
With big-bores done, let’s look to tuning these legendary motors. When I look back now, I can see that of all the Japanese two-stroke race-replicas the RG500 engine is the closest to the real thing. When the Formula 1 World Championship was held, the rules were 750cc four-strokes and 500cc two-strokes and the major engine components must have been from a road-going bike. Padgetts of Batley took the RG500 engine, fitted some of the racer RG500 parts and put Darren Dixon on board and he went and won the World Championship. Darren later went on to be World Sidecar Champion as well. I have prepared race RG500 engines for solo and sidecar racing, hill-climb bikes and cars, and sprint and drag bikes, it’s a wonder there are any still around to build into a classic bike! The RG500 engine is one of the most satisfying and rewarding engines to tune. The standard engine produces about 74bhp but when tuned and with a good set of pipes 110bhp is easily attainable. In fact an engine I tuned for a guy years ago I read in the press now produces 140bhp when fitted with all the latest bolt-on, go-faster parts, but the bolt-on goodies won’t work if the original porting isn’t right. The secret of tuning the RG is in the transfer ports. The original port windows (that is where the ports emerge into the cylinder) are far too small and are not a good shape at all, the rear transfers are little more than round holes. When tuning the RG I spend around six hours just on the transfers making them all open at the same time (39mm from the top of the barrel face) and making each pair of ports the same widths and making sure they all enter the cylinder at the same angles. The bridges between the ports are narrowed and the rear transfers are re-angled. The rear boost port must open at the same time as the other six transfers, this is important because the mixture emerging upwards from the boost port is what directs the incoming mixture from the other ports to travel up and over the combustion chamber where it is ignited by the plug before the burnt gas is drawn out of the exhaust port. Because the boost port is at an angle, when the cylinder barrel is re-bored the top of the port becomes lower. When carrying out a re-bore the rear boost port should always be raised to the height of the other transfers. I have seen cylinders that have had a big-bore conversion and the tops of the boost ports have ended up 6mm too low. The top of the exhaust port I raise to 25mm from the top of the barrel face and widen it to 39mm, the original shape is pretty good and when widened it doesn’t need of lot of work doing to the shape. I then flow and polish the exhaust port. The transfer collection areas at the bottoms of the barrels need very little work, just matching the castings to the liners. The cylinder heads I mount on a faceplate on the lathe and machine 0.5mm off. I machine the squish bands to 21º and a diameter of 57mm, to allow for future re-bores. With no step in the head and the squish starting at the head face this gives a squish clearance of around 1mm which is right for a road bike. Right, so that little lot covers stripping an RG square four engine as well as how to make a big-bore kit and tune the thing. Next month we will look at how to put it all back together again!