OzVMX Forum
Clubroom => Tech Talk => Topic started by: PEZBerq on March 06, 2011, 04:09:12 pm
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I have had the pleasure of a new (came in a plain white box) Weisco piston nipping up in my big bore husky after a few minutes riding. I hadn't done anything different re warming up the engine than on my other bikes (Mahle pistons) and was riding it with low throttle opening mostly in tight trees. It stopped while idling going down a steep hill and didn't want to kick over - damn >:(. Resulted in some pushing followed by a tow by quad back to the pits :-[ :-[ Not too much damage done so a light hone should see it back in action soon I hope.
Have since read that Weisco's are notorious for this and that they require special attention to warming up - especially when new. Something along the lines of forged pistons not being dimensionally stable compared to cast pistons and taking several heat cycles for the internal stresses from the forging process to equalize and stop distorting when heating up.
My piston is 86.92mm dia and I can get a 4 thou feeler between piston skirt and bore near bottom of the barrel. I did find the ring gap was too tight about 6 thou ( thought I'd checked it :-[) so I have filed it back to 14 thou so that wont be an issue again.
I guess many people must be using Weiscos and have figured out how to make them work properly. What are the tricks to living with a Weisco ??? ??? ????
Thanks
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Who did the boring? With a cast one they usually makum a bit looser, did your man do that? Has he had much to do with bike stuff?
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Old Wisecos were notorious for expanding a lot in the first few heat cycles, causing seizure. The solution is to bake them in the over for a while before getting the cylinder bored. I can't recall the temp/time, but I know that Lozza has posted it up in the past.
Wisecos made in the last decade or so, don't have this problem. It won't help you if you got it in a plain white box, but the rule of thumb is "old logo on box = old type piston, new logo = new type piston".
More generally, forged pistons (like Wiseco) will expand more as the heat up, compared to a cast piston. Newer materials minimise the difference.
This means that a forgie will need marginally more piston-bore clearance, to get the right clearance while operating.
Most car engine machiners are insistant on giving them way too much clearance anyhow, so its rarely ever a problem.
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I have had a wiseco piston in my 83 IT250K for a year and a half now with no trouble,no crazy warm up procedure or anything silly, but i did send my barrell to be bored by them aswell as this must be a perfect match, wiseco have made pistons for many a year and have supplied race teams world wide with a good reputation,all i can say is the boring process is as important as just buying a piston.
cheers Dale.
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Have ran wiscoe's in all my rebuilds with no problems. I would be more inclined to check ring to bore clearance's
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Oldfart, do you run the wiseco kit , as in piston, rings and smallend bearing? just out of curiosity.
thanks Dale.
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G'dat lads .
,old school guys will tell you from past experience that wiseco pistion have a tendicy for picking -up when running -in ,some say it's the way you run the bike in .ie. duration .
bore tolorence, throttle use , temp.,mixture,etc. It has been my experience with them that they require pulling -down and eliminating the high spots on the pistons by relieving inconsistencies ,sometimes this process has to be repeated .The affects on the pistons from this prcess is minimal . regards Jimbo
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Kit form , as you have described. Rebores are done by Serco ( Allan)
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Most quality forged pistons come with a sheet showing clearances.
Have done a few drag car engines and .006 at the bottom of the skirt on a 4" bore is standard.
Don,t forget that this is in a watercooled engine and that the engine is NOT expected to run for long.
Had a 402 small block chev in a commadore go for 2 years with no problems and it was getting quicker the older it got.
Surprised you had a problem and I understood that forged pistons were more stable than cast.
Was it in the white whale ?
if so bummer.
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Maybe it was a case of not enough fuel/oil until fully run in.
I know there will be some out there that say, der.
You mentioned going down hill and off the throttle, that suggests high revs with no throttle.
Our own JimG had his early YZ 125 nip up several times at last years DT titles in Griffith, every time was at the end of the main straight while he had backed off.
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Oldfart, do you run the wiseco kit , as in piston, rings and smallend bearing? just out of curiosity.
thanks Dale.
I have in both two stroke and four, but they were air cooled.
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Yes my rebore was done at Serco,i have run the same fuel mixture from the first revolution till now(not saying what mix, thats another whole debate on it's own)I was told to use up to 80% throttle for the first tankfull but do 20 min heat cycles if i could,i was at castella so it was perfect to do two laps then rest for 20 mins,the bike has settled in beautifully and i have noticed top end power has freed up well.On a downhill i always just clutch for a second and give a blip on the throttle to keep a bit of fuel in the top end.
cheers Dale.
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Yes it was the white whale Mike - 500 auto. Hence going downhill it was idling when it nipped up.
Sounds like I should have taken more care in the running in process - never bothered much with that before on other bikes. Using 3% Castrol TT btw.
The bike was not rebored - just honed by the boys at Serco. I fitted a fresh piston as the original Mahle looked very tired - something evil happened to it in a past life I suspect.
Will take it to Serco for a look see tomorrow.
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Surprised you had a problem and I understood that forged pistons were more stable than cast.
"Stable" can mean a lot of things.
If you overheat a forged piston, its less likely to fail catastrophically.
They are also mechanically stronger, so they'll withstand detonation better.
But they do expand more with heat and require large clearances to avoid seizure.
As I said earlier, a modern forgie is much better than an old forgie in this regard, but still not as good as cast piston.
All of those rattly old Volvos and air cooled VWs that just keep running have (factory fitted) forged pistons - they're not really that badly worn out, its just that the pistons rattle around a whole lot until they warm up.
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i use wiseco's.
Agree with above, clearance for sure and going lean when no fuel was gong in kept the temp climing.
strange you say it was only honed and the old piston was flogged, i wouldnt be surprised if it had a thrust area and your bores a bit oval for it
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Our own JimG had his early YZ 125 nip up several times at last years DT titles in Griffith, every time was at the end of the main straight while he had backed off.
That will be to lean in general usualy the pilot and the main.
Just bake at 200c for an hour and then take it to have it rebored.
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dale,
We had a small Penton , Hodaka , Cam Am shop back in the early 70's and did our own boring.
We found that Wiseco pistons made at that time were prone to sticking if set up to Wiseco's specs. End result , we would only fit one to a cylinder on the customers request.
10 years ago we started restoring and riding vintage dirt bikes in the woods and at the yearly ISDTRR we have on this side of the pond. Pistons ( NOS ) were in short supply for one engine we were using so we called Wiseco to see if they could come up with a cross for another brand they made that would work.
Wiseco had a Tec line back then and we brought up the sticking problem we had experienced. The Tec went into why the old pistons would do this and what they had changed over the years to correct the problem.
Wiseco in 2001 was using the same alloy they used in their pistons in the 70's. The difference in a new Wiseco was in the way the piston was machined.
Pistons are egg shaped front to back and tapered from top to bottom. This is ( we are told ) done to allow for the different rates of expansion caused by the differences in the amount of material present at any one spot in a given pistons design.
The pistons are " Cam " Ground to a shape that approximates round when they are at temp and under load.
New Wiseco's have worked well for the Penton , and Hodaka guys here in the US in competition .
If all you can find are the old run pistons then fitting them looser is an option but still may give you trouble in the end.
Best thing we have found , you can do in a home shop , is run the engine in with a good high output fan blowing over it for 10 or so minutes . Pull the top end down and hit the high spots on the pistons with 400 grit wet dry. Reassemble and do it again until you no longer find signs of high spots.
Its something the Road Race folks had to do years ago. Its a pain in the lower extremity's but is is do-able with limited resources and if no other pistons are available.
hope this is some help,
john d.
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Great input Jonn D, i have grown to trust my mechanic(and that does'nt come lightly)being Shane Ireland who owns Slipstream Motorcycles here in Melbourne,he is slightly older than me and is a Yamaha trained mechanic but also is not afraid to mix it with a few Victorian road race titles under his belt plus being a sponsered KTM enduro rider a few years ago now,was sponsered by JD motorcycles and yes Jeff cofirms this, but anyway he says he has worked on alot of bikes over the years(race teams etc) and says the piston failures from wiseco you could count on one hand,thats a shitload of bike racing in 25 odd years ,plus he uses wiseco in his current KTM 450 taken to a bigger size, and he said "i would not sell you a piston i would'nt use myself, your bike is ready to thrash with a few simple breakin proceedures" but that was only the heat cycle for a start and just use up to 80% throttle for the first tankfull and i could honestly not be happier with the way my IT has run in the past year and a half.
cheers Dale.
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I've gathered from this thread that 'pistons aint pistons' to coin the ole Castrol phrase.
I have picked up two Wiseco piston kits of the same bore size for my YZ125G. The first kit is an old version and even has 'spiral' clips. The other has a newer logo and it is bevelled (?) between the ring and crown, but still carries the older model numbering of 435P2 (now 435M05650).
Based on what's been said in this thread I gather it would be unwise to replace a worn piston with another from a different era? Should I toss the older one?
The other issue is clearance. The sheet with the old piston says .038mm for 52 -60mm bores and newer one says .051mm.
I guess it would be best to stick with the latest #435M0XXXX versions from now on
cheers
(http://i380.photobucket.com/albums/oo249/vmxwoody/DSC04496.jpg)
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JUst GIve it clearance.
and dont use those Loop clips, use the later circle clips, there is a large chance the wire that goes back into the centre will vibrate -break off and go into the motor.
if the piston doesnt thave the tear drops in the gudgeon area like the one on the right, just grind a little spot there so you can fish out the normal circle clips out easier.
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.038 is .0014 thou and .051 is 2 thou.
Yep that might just nip up real easy.
Those are watercooled clearances. ::)
Did my trade as a engine reconditioner , started at Bryant Engineering in fortitude valley.
Have kept my hand in ever since.
My favourite is the boring bar.
Have bored thousands of engines.
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thanks guys,
so Mike52 what clearance do you recommend for an aircooled 1980 YZ125G 56.50mm bore. the later Wiseco pistons suggest to go for .051mm.
also when did drilling the three lube holes in the piston become common practice?
cheers
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thanks guys,
so Mike52 what clearance do you recommend for an aircooled 1980 YZ125G 56.50mm bore. the later Wiseco pistons suggest to go for .051mm.
also when did drilling the three lube holes in the piston become common practice?
cheers
Cast or forged?
.051 [ .002 thou] across the bottom at right angles to the gudgeon on a cast piston would be fine.
Give the forged one an extra .001 to .00125 thou at the same place.
Have never understood the metric system.
Really prefer imperial stuff because I can picture it in my mind.
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= a poofteenth or beesdick, banana hair all very close in size
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A mickey whisker.
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= a poofteenth or beesdick, banana hair all very close in size
A mickey whisker.
Which are all the same as a cigarette paper of course. ::)
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I think its just the Wisecos on the blue boxes that need to be pre heated. Im sure i read that somewhere on here. Never had problems with Wisecos. before.