5 replies to this topic

[liirge]

Right i've had a message from my machine engineer, asking what clearance i would like between the Piston and the bore.
I looked on the box and it said MINIMUM CLEARANCE: 0.0015.
So is that the clearance i want, or do i add a bit to it?
Cheers in advance
Alastair

[liirge]

Ive found some info for 73.5 Pistons, that the ideal Clearance is 0.003".
Is this the same for my smaller 998 +120 thou pistons?

[jaydee]

0,003"=0,0762mm which is approx the clearance used in the later 1275s.
Isnt that too much for a small bore? Wouldnt it be about 0,065mm for a rebored 998 block?
Hopefully someone who has built a 1098 can give a good reply.

[liirge]

AC Dodd recommended a 0.003" clearance, oh crumbs i hope your wrong jaydee i told the machinist to do the clearances to that

[Wil_h]

3 thou should be fine. I run 4 thou on the hillclimber, but I had issues with the pistons tightening up a bit in the bores. 4 thou is a bit big for a road car as it breathes a lot at tickover.

[Cooperman]

I use 0.0025" to 0.003" on 1275's and 0.016" to 0.002" on 998's. That seems to work fine. For an 850 it is quoted as 0.0006" to 0.0012" in the original workshop manual. For a race or rally engine of 1275 or greater it does no harm to go 0.001 over the quoted sizes although, of course, they will burn a little more oil and need re-building sooner.