TWO-STROKE ISSUES
Stan Stephens highlights some issues he’s had!
Ioften have to sort out some pretty horrendous issues with motors before I can even get inside and see what I need to do. Here are three I’ve recently come across.
Yamaha RD250LC crankcase repair
A customer arrived at my workshop with his son to deliver an LC250 engine. The son had bought the LC for his dad (I wish my sons did that sort of thing!). Dad didn’t sound too grateful though, he said the bike was a dog and the engine certainly looked it. They wanted the engine all rebuilt with genuine Yamaha parts and all the steel parts zinc plated in that olive green. When I stripped the engine it needed absolutely everything doing to it. The bike had had many different owners and none of them were mechanics. Every gasket face had hammer marks or lever marks, most threads were stripped: not nice but it was 30 years old and many engines that I get in aren’t in a very good state. I suppose that’s why they are brought in to me! The worst piece of bodgery though was one of the barrel studs. When I undid the head bolts one of them just went round and round. When I tried to lift the head it wouldn’t budge, I had to remove both barrels and head together. The stud that the head nut wouldn’t move on came out with the barrel and on the end of it was a big lump of body filler. I thought that I would have to fit an insert when I rebuilt it. I did the full estimate for the rebuild and renovation and it came to the highest figure for an LC that I have ever had to tell a customer. He wasn’t happy but he said to go ahead. Before I took the crankcases around to TSR to be vapour-blasted I thought I had better do the insert to the damaged studhole. It was a lot worse than I had imagined. When I removed all the filler the 8mm hole was a huge 20mm jagged hole with a broken off drill in it. I couldn’t now tell the customer that his cases were scrap! With a lot of careful work I managed to remove the broken drill but I was left with this massive hole where there should be an 8mm studhole; this was going to be a salvage repair. I drilled the hole out until it was 22mm and round. I made a 22mm solid dowel and tapped it into the hole, leaving it a couple of mm from the top. I then took it to a skilled welder friend who welded it in. I now had to skim the top of the crankcase on the mill. First I had to remove the other seven studs – the first one came out but the others wouldn’t budge, I know how much effort an 8mm stud will take before it snaps and these were definitely going to snap and that would have opened another can of worms, six cans actually!
I did an article in CMM a couple of years ago which I called ‘Needs Must’ – in other words not the correct way but sometimes you have to get around a problem. Instead of skimming the case on the mill I had to carefully file down the weld but make sure I didn’t file the rest of the gasket face. So that when I was filing the weld I didn’t damage the opposite side of the crankcase mouth, I put a piece of emery cloth there. The emery was tough so that I could file the weld but not the case. When the surface was almost flat I removed the emery and finished the job carefully with a fine file, I then measured and marked out where the studhole should be and drilled and tapped it. There was a little porosity in the weld so I think I will be putting a little gasket sealer on when I rebuild the engine.
Yamaha RD500LC saga continued!
Some of the articles that I write cover problems or modifications that more than likely none of our readers will encounter but you might come up against similar problems and my articles may show a way of getting around them. In CMM’S October Issue 348 I covered the fitting of large V-force reed blocks to an RD500LC engine. On the rear cylinders the reed blocks are located in the cylinders, I had plates welded either side of the inlets so that I could enlarge the inlets. I mentioned in the article that I would have to relocate the small water pipes that go into the rear cylinders by the inlets, here I have shown a photo of their new location. The original job was to remove the troublesome 680cc conversion that someone had fitted and replace it with a SS tuned 500cc top-end. The 680cc top-end ran without powervalves but my
customer wanted the 500cc to run the normal powervalve system. On an RD500 engine the front reedblocks are located in the crankcase and when the engine had the 680cc top-end on I had done away with the small double reed block housing and made two separate blocks for the large V-force reed assemblies. When assembling the engine a problem showed up. The powervalves are operated by a pulley, which as it is turned by the cables from the powervalve motor pulls and pushes two rods which turn the powervalves. The pulley is mounted on a tower which is bolted to the centre of the double reed block housing, but I had done away with the standard double housing. It is fairly critical that the pulley is in the exact right place so I had to come up with a plan to remount it. I still had the original double housing, so I machined it until it was 10mm thick and then machined 10mm off of the new large inlet blocks and refitted it so that the tower still located in its original position. I then fettled it so that the V-force reed blocks fitted. The pictures show it a lot better than I can explain it. It was a simple remedy to what I thought was going to be a major problem!