RR classic 3.5 ltr rebuild QC

[Bobs Project]

My local machine shop unable to garauntee consistant piston to bore clearance (0.0007-0.0013) for 3.5 ltr rebuild. Says tolerance unachievable??. New one to me...mechanic 45 years... never heard of it. Anyone have any thoughts on this?

[Pedro_The_Swift]

I thought that was the idea behind line boring?

the block must be twisted like a pretzel,,

[roverrescue]

Is it because you have stated
0.0007-0.0013

Which is 7/10,000 I assume of an inch
Or 0.017mm to 0.033mm
Which is crazy tight tolerance?


Ten times the tolerance will make a difference to capability

Steve

[Mercguy]

First:


what type of piston? forged / cast / hypereutectic?


piston to bore clearances will differ as will ring end gaps.
typically for a 'performance' type hypereutectic alloy cast piston a bore clearance of 0.002" is good. I think your tenths measurement is just a typo. The machine shop should have simply called you and asked, instead of being numpties about it. Any machinist worth their qualification would have picked that in an instant.


Check the suggested end gap for the top compression ring with the ring manufacturer as hypereutectics typically have a bigger end gap than others (cast/forged).

If in doubt re bore clearance:
heat the piston to 220 deg C

measure top diameter above compression ring, measure diameter below oil control ring.
measure again at room temp.

thermal expansion rates are typically 0.01mm per 100mm dia. per 100deg C for a hypereutectic. nice and metric !


The coefficient of thermal expansion for eutectic is 0.0000214 mm/mm/degree C and for hypereutectic it is 0.0000196

should be relatively easy for your "qualified" machinist to work that out (hint, most of them use a computer)

[tonguewink]

[goingbush]

I always gave the box of pistons to the Machine shop & they bore to suit. Never had an issue.

[Bobs Project]

Apologies...my mistake, could have explained a bit better. I was referring to the factory spec (.0007"to.0013") as per the factory workshop manual, for the standard OEM piston for that engine. To put it another way...the specified piston to bore clearance is...1 thou +/- point 3 of a thou...pretty straight forward I thought but the machine shop guy reckons he can't do it??

[Bobs Project]

I always gave the box of pistons to the Machine shop & they bore to suit. Never had an issue.
Thanks for response..
Yes... Normally no worries, but factory clearance spec is small with tight tolerance... Nothing worse than a noisy piston in a new motor... Might just have to find a better shop.
Best regards

[POD]

Yep the tenths is not a typo, I rebuilt a Leyland 4.4 for a conversion into a RRC quite a few years ago and the engine had to be re-done because of piston noise, the spec was .001" piston-to-bore clearance and when I stripped it down the clearances were up to about .0025". The machinist admitted that he had not looked up the spec but had done it to the sort of clearances he was accustomed to with other motors. He supplied a second block and did the job again to fit my already-purchased +010" pistons. After it was re-done properly to that spec it was nice and quiet. It's a tighter tolerance than people rebuilding most older engines will be accustomed to, it's better that he's told you he can't do it than to do a near-enough job and it be no good. He doesn't want to be responsible to supply you a new block if he can't hold to tenths over 8 bores.

[Bobs Project]

[QUOTE=roverrescue;2908784]Is it because you have stated
0.0007-0.0013

Which is 7/10,000 I assume of an inch
Or 0.017mm to 0.033mm
Which is crazy tight tolerance?


Ten times the tolerance will make a difference to capability

Steve[/QUOTE
Apologies.. Should read..0007" - 0013" (that's inches not mm) This is the specified clearance range that ole mate says he is unable to achieve.
Regards