ALLEN MILLYARD
Our Allen is nearly there with his RC374 replica – but first he needs to sort out the clutch.
The genius is sorting his six’s clutch!
I’m well on the way to completing my RC374 six-cylinder engine. All the major engineering is complete and have carried out several dry builds to test individual systems, so I’m now getting ready for the first wet build with gaskets and oil. Due to the uneven shaped sump on the FZR crankcases I decided to make up a simple engine stand to support the engine while I build it. This will be used later on to mount ancillary components like the ignition coils and battery for the first start test. When I replaced our bathroom last year I had kept the old angle iron bath support rails which were a perfect size to make the engine test stand, so after a bit of cutting, drilling and welding I soon had a suitable test stand made. I then started to lay out all the engine components ready for assembly and was checking the clutch parts when I thought to myself: ‘…this is a wet clutch and it really needed to be a dry clutch like the one fitted to the original RC174 six-cylinder engine’. Racing bikes are usually fitted with a dry clutch to prevent the organic clutch plates contaminating the oil if they slip and burn, and also it makes it quick and easy to change the clutch plates if required. The clutch normally works sealed inside the engine and is attached to a gear that meshes with the crankshaft and is therefore flooded with oil during operation. What I needed to do first was separate the clutch basket from the drive gear; the clutch basket is riveted to the drive gear through the centre of six transmission damper rubbers so I drilled out the rivets
and carefully separated the parts. The clutch has an inner hub and outer basket; each would need to be modified and sealed relative to each other, the clutch outer basket would also need to be sealed relative to the engine crankcases via a new cover plate to prevent oil leaking out. The first thing I did was to make a new crankcase cover plate from 10mm thick aluminium that would fit in place of the old clutch cover. To mark out its shape, I put the old clutch cover onto the aluminium plate then used a scriber to draw around the outside. The new cover was cut out using my bandsaw, closely following the scribed line and once cut out the edges were cleaned up with a file and emery cloth. The old clutch cover was then clamped to the new cover plate to use it as a jig to drill the securing holes, and I also chain drilled several holes to remove a section of the plate roughly in line with the gearbox shaft. The new cover plate was then screwed in place on the engine so that I could establish the centre of the gearbox shaft. I did this by clamping a scriber to the gearbox shaft then gently rotating the shaft one revolution so that the scriber made a concentric scribed line on the cover plate. The cover plate was then set up on my lathe in a four-jaw chuck, the scribed line trued up and a 110mm stepped hole machined to fit an oil seal. The rear of the cover was also recessed on my milling machine to give clearance for the drive gear. The cover plate required one more hole to be drilled to fit an oil filling cap, then I machined up a short piece of aluminium tube cut diagonally at one end with an internal thread to match the original filler and welded it over the hole at the top of the cover plate. The next job was to extend the clutch basket and clutch hub at the rear to allow it to pass through the new cover plate and attach to the transmission gear and gearbox input shaft. A recessed datum face was first machined on both existing clutch parts then I made two ‘top hat’ style extension tubes that fit inside each other with an oil seal located in a recess on the outer tube and sealing on the inner extension tube. With the machining complete, the clutch basket extension tube was riveted onto the transmission gear incorporating the transmission damper, and the inner extension tube was fastened to the clutch hub by drilling six holes and using high tensile countersunk cap head screws. Finally I had to block up the old oil feed to the clutch plates that ran through the centre of the gearbox shaft, this was easy to do by welding the small drilling in the clutch release rod. The finished clutch was trial assembled and was seen to disengage and spin freely when operated with a spanner on the clutch release lever. The outer surface and welds on the cover plate were roughed up a bit with my Dremel and carbide rotary burrs to make it look like it was a casting. A replica clutch guard was then made out of 2mm stainless steel sheet and was screwed to the cover plate with M5 screws.
During previous dry builds I had been using two old FZR head gaskets spliced together to make up the thickness but I had to make a new one for the final build and decided to use copper like the original RC174 head gasket. I bought a roll of 0.3mm thick copper foil and cut out a new gasket with tin snips and hole punches. The new gasket was trialled onto the barrels while fitted to the crankcases to check that I had got all the stud and bore spacing’s correct and I was pleased to see it fitted just right. The copper gasket was then annealed to make it soft and malleable by heating with a blowtorch until it was cherry red then dropping into a bowl of cold water. I was finally ready to build the engine which was by now a straightforward assembly task, having built it so many times dry. I started by placing all gearbox components and the crankshaft into the lower crankcase, then applied a thin smear of gasket cement to the joint faces and tightened the upper crankcase in place followed by the barrels, pistons, head, cams, clutch etc. I left off the cam cover so I could check oil flow. I filled the sump with oil and carefully turned the engine over with a spanner on the crankshaft end nut to make sure it would complete two whole revolutions freely. I then connected the negative battery lead to the crankcases and held the positive lead to the starter motor terminal to spin the engine over for a few seconds. Almost immediately I could see oil flooding the camshafts so I stopped and fitted the cam cover. I continued to spin the engine over on the starter in short bursts for around 10 minutes, checking for leaks and feeling for compression on each cylinder with my finger over the plug holes. Each cylinder forced my finger off with a similar loud squeak confirming it had adequate compression. With such a positive result I decided to proceed to the next stage, which is sourcing,and fitting six carbs and the ignition system ready for the first start up on the test bed.