Piston clearance

[IanHood]

Does anybody know what piston to cylinder clearance I should be aiming for with a new Omega road piston in a 450 Desmo?

[themoudie]

Aye Ian,

I could quote the figures from the Clymer manual, but I think that if you would care to wait a wee while that Nigel might reply, with chapter and verse, specific to the Omega piston.

As well as checking the piston to cylinder clearance, you also must check the clearance between the rings and the piston ring lands. Also, the ring end gaps, after inserting them into the bore, need to be checked and adjusted as necessary.

Good health, Bill

[Jon Pegler]

I believe that the piston clearance for an Omega piston should be 0.003 - 0.0035 thousandths of an inch.
Omega pistons are usually manufactured undersize to allow for the clearance, so that, for example, an 86mm piston is correctly made to fit a bore of exactly 86mm.

[IanHood]

Thanks both. Think I will wait for a view from Nigel (since he supplied it) - I was assuming about 25-35 thou based on my experience with other rebuilds but best to be sure.

Ian

[IanHood]

...and the answer is 30-35 thou

[Duccout]

Ian, you do mean 3 -3.5 thou , not 30-35 thou?

Colin

[IanHood]

er yes, slaps* head, 3-3.5 thou

As the piston would at 30-35 thou

[IanHood]

Here's Nigel's full advice for the Omega pistons:

Bore Clearance - 3 to 3.5 thou (0.075 ~0.090mm)
Ring Gaps must be checked. Recommended gaps as below, ideally getting larger as they go down the piston. The gaps may
be slightly larger but not smaller.
- Top 12 to 14 thou (0.3 ~ 0.35mm)
- 2nd 14 to 16 thou (0.35 ~ 0.4mm)
- Oil (Expander ring)16 to 18 thou (0.4 ~ 0.45mm)
- Oil ( 3 piece ring) he rail gap is not that critical but should have a minimum gap of 15 thou (0.38mm).
- Oil rail gaps should be 90 degree to the expander gap and 180 degree to each other.
Valve to Piston clearance - We recommend that valve to piston clearance is checked on all engine builds regardless of
whether it is standard or modified due to the possibility of variations in various components.
Minimum Inlet valve to piston clearance – 0.080” (2mm)
Minimum Exhaust valve to piston clearance - 0.100” (2.5mm)
Minimum Radial clearance of valves to valve pocket – 0.040” (1 mm)