Yoshimura 60.25 GPz550 kit piston clearance?

Yes you measure piston clearance differently with cam ground pistons.
They are ground in a different shape so when they heat up and expand, they will be "round" ....to compensate for the heat conduction around the pin area.
Also, the top land of most pistons is a smaller diameter then the skirts so you should pretty much try measuring the skirt clearances from the bottoms of the cylinder. Pin area and 90 degrees away...

In 1972-73, I ran 12:1 Hepolite pistons in a stripped down, street drag bike 750 Triumph, with a mandatory oil cooler.... I had to use an serious octane booster on the street with temps over 80 to 90 degrees! Hotter then that and I would not ride the thing much at all.
(This would be more like a naked, cafe' racer bike these days)
However, I found that a trip out to the local small general aviation airport netted me a couple 5 gal cans of 100 octane low leaded aviation fuel for about $0.50 to $1.00 a gallon more the the best high octane car gas I could find at the time.
It never knocked on that stuff.
Even when mixed 50/50 with the cheaper 91-93 octane car gas and the ignition timing rolled back a few degrees, it was good.

From wiseco's website FAQ:

05. There is no piston to wall clearance on the box label, what is the piston to wall clearance for my application?
A.
Wiseco Powersports pistons are designed for optimal piston to wall clearance for the specific bore size of your engine. For example, your 125cc dirt bike engine is 54mm bore, the Wiseco piston is designed just for that bore size, and does not require any specific adjustments to fit properly.

Some applications, such as higher stressed race engines (or nitrous injected), forced induction engines (supercharged or turbos), do require additional piston to wall clearance than Wiseco has designed into the part. These are special applications that are normally built at high performance engine shops. These builders are familiar with the needs of these engines, and can help establish the needed engine set up parameters including piston to wall clearance. As a general rule, any high stress engines that include nitrous, turbo or supercharging, up to high pressure blowers require an additional .001 to .003" clearance.


Pretty useless info there. Anyone know what alloy the older wiseco's were forged from?

I've emailed Yoshimura USA, got an acknowledgement within an hour but am awaiting some numbers from them soon.

My Wiseco 1075 pistons are plus or minus, .0013" (.035mm) of each other!
I had the machine shop bore, hone and mark each of the sleeves (installed) for matching each numbered piston, based on the diameter of the "thrust face" area, 90 degrees to the pin.
Basically front to back of the piston and slightly below the oil control groove, which I believe was the widest diameter.
In normally aspirated engines, (street hot rods too) you look for around around .003"-.004" skirt to cylinder clearance at the widest piston diameter.
Air cooled engines really need to be set at the piston manufacturers specs!
And about .004" top compression ring gap for every inch of bore diameter, with slightly more gap for the middle or second compression ring.
By the way, accidentally bigger ring end gap in either ring is better then too small there!
People stress over this but it is not critical if larger then the ,004" per inch of bore as long as it is not something crazy like .040"-.060".
Even then I am not so sure it would make big difference in a street machine.
I am running about .014" to .018" gaps in mine.

did you measure the variation in these wiseco pistons your self mission?
i have a small engine machine shop and have machined thousands of cylinders to fit wiseco pistons and haven't found a variation.
i have heard everyone talk about them being different but have never found a discrepancy.
wiseco has a million + dollar qc machine that checks each piston.
hard to imagine that kinda variation getting past.
the widest diameter of the piston is down on the bottom of the skirt,
When measuring for piston to wall clearance, measure at widest point of piston skirt,
90 degrees from wrist pin hole.
leon

The machinist spent around half an hour with me, I bought 15 pistons and two blocks in...

The (used) Wiseco were around I thou difference, a block can be bored accordingly.
A member here and a good friend of mine donated some other (used) pistons but they were a lot more worn - they came from a race bike.

The used yoshi pistons were up to 4 thou difference, again the block had been bored accordingly.

The machinist told me whilst he liked the modern Wiseco design of thin rings and less piston skirt he thought they were less durable over time, this was his experience on tuned car engines.

Should I ask for any extra champher on the barrels?

The yellow 81 550 gpz in my avatar was originally fitted with the yoshi pistons, I remember finally getting it running, taking my hand off the bars to push the choke in and it nearly had me off the back! big power everywhere. Just to be honest 20 years ago i had a lot to learn, getting just a bit better at this now, mainly learning from my own mistakes.

The champion moriwaki team were getting f1 superbike grid qualifying times with a tuned 600 GPz550 .

Sorry to keep bumping my own thread, and (sincerely) many tanks to those who have contributed. Just I need some numbers asap, I have a nasty top end rattle which isn't going to last so i need to rebuild this again. I prefer to get a consensus of opinions.

plus 1 plus 2 mm OS stainless valves in ported head



Self indulgent bike porn....sorry



this one is the money shot

531blackbanshee wrote: did you measure the variation in these wiseco pistons your self mission?
i have a small engine machine shop and have machined thousands of cylinders to fit wiseco pistons and haven't found a variation.
i have heard everyone talk about them being different but have never found a discrepancy.
wiseco has a million + dollar qc machine that checks each piston.
hard to imagine that kinda variation getting past.
the widest diameter of the piston is down on the bottom of the skirt,
When measuring for piston to wall clearance, measure at widest point of piston skirt,
90 degrees from wrist pin hole.
leon

Yes. I measured them with my own mic set. And what I meant by that was that the difference between the smallest dia piston and the largest dia piston was .0013". The other two fell in between those two outside dims.
************
OK you might have me here, but I just went through my old photos and found the piston pictures.... as written on the domes, one piston says +.001 1/2". That was kinda dumb but... I think that means 1 and a half ten thousandths.
Written on the box with all four pistons is 71.9mm/2.832" +- .0013".
Now I think that means i wrote they are all within .0013" of each other and I might have measured them at about .1mm under 72mm and that would be about .003" clearance if the bore was a perfect 72mm/2.8464".
Hope I got this right 'cause now I 'm gettin' worried... LOL.
Regardless the machine shop fit all four pistons and cylinders with proper markings showing which piston goes where.
I've always been under the impression that the Wiseco pistons are cam ground and tapered.... not correct?

So 3 thou on a 61mm forged wiseco, 4 thou ok. What is ideal for a new bored block - with used pistons?

Should i ask for 3 thou in the absence of any other info?

Compared to stock, the wiseco are heavier, so are the oversize stainless valves. there are guys spinning these around to 13k with wisecos on the gpz550 forum, for my purposes i want a fast road tune with more midrange. I'll use the big valve head and 0.5mm base circle skim kawasaki gpz cams.

I have a set of megacycle x3 cams for rpm power but they might be too wild for this, I'm saving them for the yoshi kit. the yoshi pistons were as light as stock.
Wes Cooley yoshi superbike article quotes clearance down to .00015....hence my
concern at this .004 thou number for the (smaller) cast pistons. Hopefully Yoshimura will have a definitive answer for me.

I just measured all mine for the fourth time.
I have found that cold, all four pistons are clearanced at right between +.0035" and .004", even though NONE of them measure the same diameter piston nor are the bores exactly the same.
As I previously stated, and just confirmed, there is a +.0013" difference between the smallest and largest piston and ... the piston skirt clearances at the widest measured part of the pistons is more then .0035" on all and a max of snug .004" on one.
End of my quest....

OH and do not forget... that .003" to .004" is with one side of the piston skirt slammed against the cylinder wall so your actual skirt to cylinder clearance when the piston is floating and centered in the bore (not very often, LOL) is actually .0015" to .002" all around the skirt!!

wiseco pistons are cam ground and tapered.

leon

ok I'm going 3 thou bore clearance on the forged wiseco 61mm kit, this is what I'm going to be running with now, plus a big valve head and .5mm base circle skim GPz550 cams. I have a megacycle x3 set but I think maybe too wild for this with the added weight of the oversize stainless valves. On this I'm tuning for strong midrange.


Yoshimura USA rep was very helpful and replied to my emails promptly, unfortunately they have not sold the GPz550 kit for over 25 years and are unable to help further. Conventional wisdom is 4 thou, i'm going in at 2 thou total, influenced by the Wes Cooley Yoshimura f1 superbike article. I have two 550's. so first job will be the wiseco kit.

600 posts later I think i know where I'm going on this, thanks for your help everyone who contributed. Over and out.

break it in at varying rpm(don't hold it at the same rpm for long stretches, keep the rpm moving)for a tank tank and a half and you won't have any problems with that setup.

imho fwiw.

then start asking it for it's life . :evil:

keep us posted,
leon

Tyrell has ended this thread. He tells me he will start a new one with more info later.
Steve