Topic: Piston clearance for forged Wisecos

[bistromath]

I just took delivery of a set of Wiseco 40059M05850 pistons for a CB550F and had a question about clearance.

I carefully measured the skirt diameter of the original pistons at 58.50mm. They're scored a bit from seizing at least once, so the machinist recommended replacing them. However, the bores measured up tight and round at 58.52mm (min 58.51, max 58.53). The pistons I just received are set up at 58.45mm, meaning my original, honed bores are already oversize for the recommended Wiseco clearance of 0.051mm. It looks as though the Wiseco pistons were set up for a 58.50mm bore, rather than for a 58.50mm piston as Honda set.

Can I use these pistons at 0.07-0.08mm clearance? I'm really aiming for reliability here, as I don't want to go back in there again any time soon. I just wanted a little compression bump to 10:1... 

Wiseco answered the call on the first ring and say they have no problem running 0.08mm, and that the 0.051mm spec is a minimum. I figure they know what they're talking about, but wanted to get a second opinion.

[PeWe]

Forged pistons normally need more clearance than cast.

My last 836 pistons had 0.2mm without smoking, cast pistons! It must have running with a clearance of over 0.1 mm for long time with the earlier cast pistons, mostly long touring in higher speeds.

Wiseco must know whats OK or not.
JE recommends 0.0025” for many different pistons.
Dynoman 592 kit, 0.0028” 0.071mm
http://www.dynoman.net/engine/pistons/piston-install/61mm-v3.pdf

I’m sure you’ll get more answers about this.

[blacksmith1]

had 2008 wiseco seize in a modified sr 500,mates also had that years pistons and froze as well,a mate got onto wiseco and received new pistons from them and they worked well,not sure about clearances ,always thought a change of alloy mix ,the early ones for sr 's are still in bikes,not sure if they went overseas for a period or what

[fire113]

hi,


...had the same question Years ago 

http://forums.sohc4.net/index.php?topic=68372.0


Have a good one...Georg

[bistromath]

Bore specs are based upon cylinder dimension, not piston dimension. The 58.50 bore is exactly that, a bore of 58.50mm. Wiseco pistons are machined to size for the bore less the clearances. Cast pistons of 40 years ago were typically hand polished post-cast to remove flash and achieve a nominal dimension.

If Wiseco says they're good with the extra clearance, I'd take them at their word. But personally, I'd re-bore to 59mm and use a new set of Wisecos for 59mm, bored to the exact clearance on their package. Will cost you a tad more now, but save you heaps later. Perhaps you can exchange the pistons?

That's kind of what I'm thinking. Sounds like I'll end up falling at the upper end of safe. The build has been sitting for a couple of years now, so a few more weeks to get some peace of mind won't hurt. It's going to be a street machine anyway, so I don't want the larger clearances for racing.