RD500LC WOES
Stan again on what can go wrong.
This month my article will be about an RD500LC engine I have been repairing and rebuilding. One problem I found on this engine was that the clutch hub nut had not been tightened properly and the lock washer looked as if it had been reused many times, the result was the clutch had chattered on the shaft and almost stripped the splines in the centre. There was nothing I could do to repair it. I ordered a new one from Yamaha and sure enough they were no longer available, so I was reduced to searching for a second-hand one on ebay. I located one but one of the pillars that the clutch springs locate around and the spring tightening bolts screw into had broken off and wasn’t there. The job of repairing this problem applies to lots of bikes as many suffer the same fate and I have seen some horrible bodges. If you still have the broken off pillar the repair is simple. Usually the pillar breaks at the end of the thread but it is a good idea to run a tap right the way through. Most have 6mm threads, drill out the back of the clutch hub and to make a neat job counter-sink the hole. Put the broken-off pillar back in its place and screw a 6mm counter-sunk Allen-screw with Loctite on through the back of the hub and into the pillar, job done. If, like my ebay clutch, the broken off piece of pillar is missing it makes it a little harder job but is still fairly simple. The area where the pillar is broken off I machined flat. I then made a pillar from a piece of alloy bar and drilled and tapped it to 6mm and like the previous repair I drilled and counter-sunk the back of the hub and bolted the new pillar in place. In a previous article I said that the RD500LC engine suffers from a design fault in the right-hand main bearings. I find that usually at around 21,000-23,000 miles the main bearings crack around the circlip groove. The reason for it is that the groove has been machined in the thinnest part of the bearing, right above the ball bearing track, a couple of mm either way and it would not have been a problem. When I wrote the article I didn’t have a good example to take a photo of but on this engine the cracked bearing was still in place and shows the problem well. Not too long ago I wrote an article about a customer’s RD500 engine that was fitted in a Spondon chassis which someone had drastically over-bored and fitted liners to take 66mm Yamaha IT175 pistons: it was a terrible job that had been carried out and there were many places where the machining had broken through and had been repaired with filler. The customer was Ian Buxton, who when we started talking, persuaded me to design and build the outrageous 1200cc V6 from two RD500 engines. The problem Ian had with the 66mm RD engine was that the person who built the engine had raised the exhaust ports way too high and had then compounded the error by not having
powervalves, in fact there was no room to fit the powervalves; the liners were so big they filled the space that the powervalves normally went in. The result was that the bike was unrideable, the dyno-graph was hilarious, it had less power than a restricted 125cc bike up to 8500rpm and then gained 90bhp over the next 1000rpm. Ian wanted me to try and spread the power out so that he would be able to at least ride the beast. After a lot of work I was pleased with the job and was confident that I had spread the power out, but by how much I didn’t know. Ian took the engine back and said he would ring with the results from the dyno. A few weeks later when he rang I must admit I had my fingers crossed, but Ian was well pleased the bike hadn’t lost any of its top-end power but had spread the power out over 2000rpm further down the rev-range. A few months later and Ian rang again, one of the barrels had sheared in half! When he brought the engine back in you could see that where the huge liners had
been fitted it had left parts of the barrel wafer thin. Ian said he had had enough of the 680cc and would I rebuild it with 500cc barrels with power-valves and tune it and he had brought yet another RD500 engine in to use for parts. His Spondon is a race bike and is fitted with large 38mm carbs and Banshee V Force reed-blocks and he wanted the 500cc to use the big carbs and reed blocks. The V Force reed-blocks are larger than the housings in the rear barrels and
break through when you try to enlarge them with a porting tool. Knowing this, I decided to plate the sides of the reed housings by welding alloy plates on. There are the small water pipes on the sides of the inlets which get in the way so I removed them, I will refit them in a different place later. So that the plates would lay flat, I ground off the raised lettering and numbers, 47X etc. With the plates welded on I milled out the inlets to take the V Force reed-blocks, I will be finishing off the inlet ports with my porting tools when I tune the barrels. On the RD500 the rear reed-blocks go into the barrels in a conventional way but the front reed-blocks go into the crankcase. I had already modified the front inlets and crankcase and made alloy housings to take the V Force blocks when it was 680cc. When this engine is tuned and finished I have an idea that Ian expects it to be as powerful as it was as a 680cc motor, it won’t be but it will be rideable and won’t shear barrels in half, anyway if he wants power, the chassis will soon be finished for the 1200cc V6!