Wiseco Pistons - Secret Mens Business?

[PEZBerq]

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

[500Fool]

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?

[Nathan S]

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.

[dalesween]

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.

[oldfart]

Have ran wiscoe's in all my rebuilds with no problems. I would be more inclined to check ring to bore clearance's

[dalesween]

Oldfart, do you run the wiseco kit , as in piston, rings and smallend bearing? just out of curiosity.

thanks Dale.

[jimbo]

 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

[oldfart]

Kit form , as you have described. Rebores are done by Serco ( Allan)

[Mike52]

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.

[Curly3]

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.

[500Fool]

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.

[dalesween]

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.

[PEZBerq]

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.

[Nathan S]

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.

[Freakshow]

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