Yoshimura 60.25 GPz550 kit piston clearance?

First thread here and belated Hello to he Forum, keep it coming, good discussions and very informative.

I've been lurking here for years but only recently signed up. I'm into early twin-shock GPz/Lawson from midlands UK.



My question : Yoshimura 60.25 GPz550 kit piston clearance?

Can anybody tell me what sort of piston to wall clearance is optinum, and what can I get away with here? I'm not looking to do racing or long distance touring, just get it running again even if it only lasts a thousand miles or so more.

Pistons are 12:1 , 3 ring (high silicon?) cast alloy Yoshimura 60.25mm 550cc to 596cc kit, I have some NOS rings and will do some basic measuring with feeler gauges. I know this isn't the way to do it here, but I don't have the mics so I just need to see whether things are miles out here, or might just be serviceable and needs more accurate measurement.



I had this running years ago rebuilt from used parts, but after a couple of thousand miles it was smoking and and blowing exhaust smoke through the breather. Again, not looking for a five year unlimited mileage warranty here. just trying to get my money's worth on the NOS rings :

If I can get away with new rings and selective fitting of seven used pistons into an already bored block I will be laughing here, I just want an idea of range of measurements please.

Conventional wisdom dictates that why bother with all the work, when buying a new parts and 'doing it right' would be the sensible way to go. I just like the idea of wringing a few more miles out of this obsolete block, pistons and NOS ring set before the last of the yoshi kits goes for the scrap bin, and I bore out to 615cc Wiseco.

pic: Wiseco XC 615cc next to Yoshimura piston

.003 thsnths cylinder wall clearance should be a safe number.

a lil tighter (.0005 thsnths)might be okay if you brake it in easy.

leon

003 thsnths cylinder wall clearance should be a safe number.

a lil tighter (.0005 thsnths)might be okay if you brake it in easy.


ok I'm just getting a 0.10 mm feeler gauge into the skirt , about .004"

This is the 'what can I get away with' part of the question. Also did the block borer maybe go a bit too wide on the clearances here? Often a cast piston expands further and has more clearance and piston slap when cold -forged pistons expand less. I read the 143 bhp yoshimura GS1000. :laugh: ran at a tight piston to wall clearance, the high silicon cast pistons expanding less apparently.

I measured with a NOS ring at top and mid section of bore and got .03mm ring gap - nearly .0012" , indicating very little block wear? The block was bored years ago and the pistons numbered, but no more than 2000 miles ago max. He gave me the choice of tight clearances -break in gently, or looser for full power soon after break in. I opted for the former. I guess pistons wear a lot faster than a block?

Without internal and bore mics I'm just trying to sort scrap metal form potentially usable parts
here, What if I run 25% over the service limit here and ,as always, not thrash it when cold until fully hot, Do I risk lower power and high oil consumption, or a castastrophic piston failure? planning to use a worked head and performance cams and good oil here for a fast road tune.

pics of piston kit etc

s623.photobucket.com/user/the-black-jack...0%20D1?sort=2&page=1

typically cast pistons expand LESS than a forged piston.

i wouldn't be scared of the .004 thsnths

considering that .010 is the next oversize and most cylinders give a .008 thsnths max wear.

you pays your entry fee's and takes your chances !

leon

Sorry to bump an old thread, but any advance on these cast yosimura 60.25mm bore/clearance numbers?

Now I'm getting 4 thou micrometer check, about what my feeler gauges told me. This block is at a machinist now and he wants to know the clearance specs.

I'm thinking maybe 4 thou is a bit wide, and will then bore out to the next size up wiseco, Then bore a another block to the 60.25 but maybe less clearance for my other 550 project.

Apparently the yoshimura pistons were a high silicon content, which also alter the expansivity, hence my clearance question...

Another question: what is a good clearance for wiseco 61mm forged pistons -this is the old type big skirt, fat rings design?

Ape did my clearance for the 700cc Wiseco's at .002 on my 650..... 3 of the top rings were at .0085 and I opened one up from .005 to match and the 2nd rings I left at if I remember right .011 to .0135 ..... only had to grind the one top.

.004" is a serviceable clearance. Have the machine shop bottle brush hone the cylinders and you should get a good many miles out of the engine.

12/1 pistons. Will it even run on this gas?
Steve

Are these cam ground pistons, and or tapered skirts???
I used to figure about .004" to .006" at 90 degrees to the pin for cam ground aluminum pistons.

Are these cam ground pistons, and or tapered skirts???


Not sure, does this affect clearance? using a NOS ring and feeler guage we have summed it at 4 thou. .. What the rebores were cut to, just seems wide to me. I read the period Wes Cooley GS1000 yoshi write up...mentioning down to .00015 clearance. I'm thinking the 60.25mm would be the smaller of the yoshimura piston range?

If I can't get any numbers out of Yoshimura, I'll just follow my nose on this and go a bit tighter than 4 thou...probably less than 3 thou. Any more opinions very welcome here.


On the 61mm forged wiseco, I've seen the 2 thou spec before, any advanc on 2 thou?



12/1 pistons. Will it even run on this gas?

True, not really what I would order today, Modest CR gives nice smooth running cool motors, I like. Just I really want to run the last out of these obsolete pistons, octane booster I expect to need.

These are the three ring road set, they made a 2 ring race only part. I had this running years ago and it was very powerful - really tourquey but setting all this up with slotted cams and a bike made of a dozen others was just a little above my skills at the time. 20 years later it is just one thing I want to finish, but do it right this time.

For the problems of high CR: detonation, heat, valve / piston clearances compared to the modest gains it probably is not worth it for a fast road motor - I know that.
I just like the idea of probably the last yoshimura GPz550/600 in the world...even if it only lasts 1500 miles.

I'm thinking of boring the 60.25 block to 61.00 Wiseco, then boring another 60.25 block maybe less than 3 thou clearance. for the yoshi kit.

Why not ask Yoshimura in California what the clearance should be for this piston?

... what I was thinking, just hoping an english language US importer will know the specs of the old 1980's kits. They are still available for some 80's bikes, just with a more modern piston design.