Topic: piston to bore clearances

[Matty_R2]

Hi guys. new to this site, after some info if anyone can help. I'm having some new +0.50 pistons put I the 400f as the old girls starting to smoke a bit, the guy who is doing the re bore for me would like to know what clearances to work to as these are aftermarket pistons and not sure if they are same standard as genuine Honda parts. would they be standard clearances or would they be different because of the size increase, also struggling to find what standard clearances are. any help would be much appreciated, regards, Matt

[Ilja]

First, I would check the workshop manual for your bike.
It should definitly show the piston to cylinder clearance, which you can safely use for a .50 oversize piston.

Guidelines:
Cast normally aspirated aircooled Honda pistons are in general - 0.035-0.045mm
Made my (turbo) cylinders with cast "cruzinimage" pistons 0.05mm, had it apart later... looked fine. Its a 65mm bore. 

Forged... depends a bit which material but in general I make em 0.06mm.
Newer KTM bikes with stock forged pistons run as small as 0.035-0.45mm on 100mm bores.. but those are hypereutectic 46xx forgings.
 
Wossner aftermarket forged pistons are typically 0.05-0.06mm clearance. For a 400 with coated skirt forged pistons I would say 0.05mm.

[Big Jay]

If ther are oem ART cast pistons, set them up at .001"

[754]

 Just to be clear,
 I picked up NOS forged TRW pistons , standard size for 750.
 They recommend .003 as I  recall. Now Being as these are the older Alloys, should I stick to .003 or can I tighten it to.0025 if I take it easy on breakin ?  Going to use a rigid hone to try to get out a thou and a half or so from a set of standard jugs.

[Big Jay]

Those TRWs had a lot of Silicon. We used go set them up at .002"