Weird screw.. fix and bike running.

The screw I had problems with in the last post gave me a bit of a headache. At first glance it looks like a normal M5 fine..ie a metric pitch between the threads and a diameter of 5mm.

I went back to the shop I bought the wrong screws from and they didn't have a correct one. I went to a few others that I knew of and nobody had this particular size. One guy was more helpful than the others and took some time to measure my bolt up, he ended saying that it was probably a 1BA thread. Well we've been metric in Malaysia for a long time and none of the shops I went to had anything which fitted (these were all shops that only sold nuts/bolts and other fasteners). A quick search of 1BA bolts/screw on ebay showed a few examples but these were quite expensive! I was looking at something like USD30 (incl s and h) for a couple of screws. I posted my problems on the 'Yamaha two-strokes 1955-1979' facebook group and got some great input. I think the consensus is that Yamaha used some JIS threads.

The next option was to call a friend of mine who had a machine shop,  as the threads were ok I figured he could weld some metal on the end and chuck it in a lathe and make the chamfer. Once done all that was left to do was level the end and cut a slot for a screwdriver. He was willing to do it but his place is not really nearby and he doesn't work on weekends.

I thought about it for a week and finally decided to take the most convenient option.. run a M5 x 0.8mm tap down the hole and hope that it would hold the M5 screw I bought earlier.

Taking the bull by the horns, I brought out my tap and slowly worked it in. There was no space for a proper tap-holder so I had to use a pair of vice-grips (mole-grips) to screw it in. This had to be done slowly and the tap needs to be withdrawn after one turn to clean the threads. I believe the holes go through into the crankcase and I didn't want to get any metal shavings in there! So and easy does it.

I used a thin rod with a blob of axle grease and poked it into the hole so that the shavings would stick to it and get drawn out. This is what comes out.

Trial fitting of the new M5 screw.

And this is most of the stuff out.. this will cause real carnage if it gets inside the crankcase as the main bearings sit just behind this.

Threads were finally done and a trial fitting of the screw is snug. It doesn't really take much load as the stator-plate is well supported on the sides by the crankcase casting, the main force will be in preventing the plate from spinning.. in this sense I think its fine.

Here it is installed. Above the screw just visible on the edge of the stator plate is an alignment lug cast into the crankcase, this ensures that the plate is mounted centrally. On some bikes the holes in the plate are slotted so you can play around with the spark advance if the contact point mounts do not give enough range of movement, here it is fixed. I will probably come back and check the screws after riding a few miles. If it gets loose then I'll probably need to use some thread-lock or JB-weld.

This option of tapping the holes to a different thread is really a bodge and is something I really don't like doing but the other option would simply take too long.

Here is a video of the bike running.

It has more compression and power than before (not surprising considering the condition of the bore previously!) and the increased compression has made the slipping clutch more apparent as it sometimes slips when kick-starting.

Unfortunately the noise in the crankcase is still loud so it will definitely need to be investigated.. currently sounds like the proverbial 'bucket of bolts' .

Will ride it a while and see.

Cheers!

Top overhaul

Well while waiting for the standard size piston purchased from ebay (shipped from the US), I decided to proceed with installing my old cylinder which has now been rebored to +1.75 with matching pistons and rings.

I'll bee needing these ..

Piston, rings, head bolts, piston pin, piston 

bearing, circlips and a gasket set.

Since the cylinder was off the bike, I decided to clean up the crankcase gasket surface, removing the old gasket material. Well this should have been easy but the cylinder studs got in the way... so I removed the studs to enable better access. Bad idea. 

The first three studs came off fairly easily with two nuts locked together but the fourth didn't want to play ball. A bit more force (yeah I should have know better!) and the stud broke off at the base!

Well I stopped for a while to look at my options. The usual course of action would be to drill down the middle of the remaining portion and then use an easy-out. I really dread using this as my success rate is less than 50%. Second option would be to take the engine out and bring it to someone who actually knows what they're doing.

Trying to put off making a decision I looked around the bike and I noticed the Phillips bolts holding the coil mounting plate had butchered heads. Since I had my tools out I tried to remove them with my impact driver.. well I made things worse! The Philips head was totally rounded out even though I was using the correct bit! Well at least this was something I could solve as drilling down Philips screw/bolts is easy as the damaged area will centre the drill bit. This went well and the plate came off and the threaded remnants of the bolts unscrewed easily.

Since I had the drill out I decided to try my luck on the broken stud. Since the break was reasonably flat I had a good chance to get the bit smack in the centre. You don't want to drill off centre as it will damage the threads in the casting! I started by centre-punching the broken-off stud and mounted a 2.5mm drill bit into my trusty Bosch hand-drill. oh-oh.. the front wheel and mudguard was preventing me getting a straight shot at the stud so that had to come off first!

 
The 2.5mm drill bit made a nice hole..centred well too and was followed by a 3.2mm bit. When it got to around 5mm deep I inserted the easy-out. (If you're not familiar with these they resemble a big left-handed screw and the idea is they will screw into the nice hole I just made and reverse the broken bit out. Nice in theory but what can happen is they break inside the broken piece and now you're really screwed as the easy out is made of hardened steel and you cant drill it out). I tried to move the easy out but it was stuck fast. I then heated the crankcase where the stud was mounted to see if that would help (actually this is what I should have done initially when faced with the stuck stud) and with a bit of force the broken stud unscrewed.. Really dodged a bullet there!

I then installed the piston, cylinder and head... damm the gasket set does not fit! I made a new base gasket from gasket paper and re-used the head gasket. One tip when cutting gaskets is to use a hole puncher at the corners. I had one made for leather which has several sizes. Once you've made the holes all thats left is to cut reasonably straight lines. In the picture below it looks weird because the paper is not flat!

When it came to re-install the coil-mount plate I hit another snag, I had bought 2 bolts with a countersunk head but these would not screw in. I then compared new with old and the threads were different! As this was late evening on a Saturday the shops had closed so I put everything away.

To be continued!

Rebored Cylinder with 1.75 oversize

Continuing on my YG1 engine.

In my earlier post I mentioned I found a NOS standard cylinder but the vendor only had a 1st oversize piston (0.25) over. I managed to find one on eBay but they wrongly sent a 1st oversize one. They were good about it and are sending the correct one but the wait is tough as its usually 4 to 6 weeks from the US to Malaysia.

If you remember I mentioned that the local bike shop said they could rebore my bike to 1.50 over..well I sent them the cylinder and old piston and I got them back 2 weeks ago. The new piston quality is not too good as the crown is pretty rough with one or two bumps from the casting process.. It also looks like the guy who did the rebore sanded the skirts a bit. Good sign as he probably checked the fit of the piston. The rings are unbranded.. They just have "trim 1" and "trim 2" marked on them.. I'm gonna install trim 1 top and trim 2 bottom unless anyone has any better ideas?

The rebore was supposed to be to 1.50 over but they ended up having to do 1.75 as the bore was too damaged to do a 1.50. Looks like it was done correctly as the piston sits square in the bore with minimal rocking.

Piston. you can barely make out 1.75. Arrow points to exhaust port. Get this wrong and the bike won't run.

Nice and new.

The skirts are sanded a bit.

Rings. also marked 1.75. Trim 1 will go in the upper position. 

Bigger bore.

I'll probably install it this weekend and hopefully get to ride it. Will keep you guys updated.