hypereutectic pistons - p-t-w clearance question - urgent

[rustbucket79]

I bet there are boatloads of engines built with zero measuring done, block bored to oversize and assembled with no issues.

The bulk of the early expansion happens in the lid, which is considerably smaller than bore size.

[BCjohnny]

All bores measure at or around 4.030" and some of them are a tiny bit smaller at around 4.0295 or so

With the clearances you're running that 'tiny bit' five tenths might make a big difference ...... if it were nominally 'big' I'd be less inclined to be bothered but 'small' is different

In the world of cylinder sizing that's a lot ....... and if you've got that variation hole to hole you can almost guarantee taper and ovality are out to lunch too

So I'd put the 'urgent' build on hold and measure each cylinder top, middle and bottom ....... and then at ninety ....... at the very least in six locations, with a tenths gauge set off the piston, to see what you actually have

Otherwise an 'urgent' rebuild might follow ...... JMO

[BillK]

b,
I just opened a box and measured one. It measures 4.029 at the "spot" but the spot is too small so I am sure it is actually smaller. The box states .001" minimum clearance.

I got to thinking about this and I think I had actually called F-M about another set a while back and they were the ones that told me the measuring spot was too small.

[Hrdlx62]

I think the "spot" is for the manufacturers checking and QC, not our mics. A lot of times the manufacturer states to finish at bore size plus the oversize and adjust for the intended usage. Thats my experience and opinion, humble or otherwise.

[Belgian1979]

Got my machine shop on the phone and they told me to look at the ticket on the box since the info was there. It says minimum clearance of 0.001" and the pistons are 4.02870. So that would equate to 0.0013" clearance which is what I measured with the coating scraped away a bit to be able to measure directly onto the blank spot.