Gap between piston and bore
Turbo Mini Forums

On a 1380 (73.5mm) what size gap is it generally between piston and bore?

Paul

I seriously doubt it!

edited,.i'll read the OP again,lol.


thought the general rule was circa 0.001 per inch of dia of piston,

top limit in the haynes for a 1275 is 0.0027, iirc

Edited by fastcarl on 31st Jan, 2008.

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What about offset bores Carl?

Warning:Stig in Training

It depends on the pistons, forged pistons tend to require great clearances than cast pistons, the manufacturer should be able to tell you the recommended clearances.

For stock rover pistons it's about:

20 - 30 thou on the crown
5 - 15 thou on the skirt

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is it not 0.05mm to 0.15mm?

Even then its a bit on the large side?

Edited by Sprocket on 31st Jan, 2008.

There was thread about this a few weeks back and the same error was made. I have mine at 4 thou, but this is an absolute maximum and probably for race only where oil consumption is not a big worry as mileage is small.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

Sorry got my self in a muddle:

2.0 - 3.0 thou on the crown
0.5 - 1.5 thou on the skirt

I use 0.06mm which is near 25 thou. Seems to work. FOr instance, I bore the 1293 blocks to 71.12mm whereas MEGA pistons are 71.06mm nominally.

website:- http://www.terryhunt.co.uk

Bore size stated in documantation is usualy the piston major diameter plus the maximum clearance, so a 1275 standard bore of 70.625mm on minimum clearance, isnt 70.625mm.

Any machine shop worth their salt, unless you tell them otherwise would take your piston, measure it and bore/ hone the block to the minimum clearnce, thereby leaving enough room for a hone if after 3000 miles the bores and rings are fucked (for what ever reason) without going over the maximum clearance.

Performance engines are different and honing to the maximum cleacance should release some hp, but would render the bore scrap if it needs another hone, unless you are happy running larger clearances than is generaly recomeneded, not un heard of.

From Haynes manual

0.074mm-0.114mm at top
0.023mm-0.064mm at bottom


Terry

website:- http://www.terryhunt.co.uk

The Haynes say different things depending on which (Metro/Mini) and which colour Haynes it is, but it's really the bottom skirt clearance that matters on a rebore/hone as the way the piston has been machined will determine the top clearance.
After much thought, research, and advice from this forum, my block is in being bored/honed at 0.06mm (0.0024") bottom skirt clearance on new 73.0mm pistons. I wanted the upper end tolerance because of extra heat from the turbo.
The machine shop didn't think my demands were unreasonable, they just said I should quote in "thou" not "mm" as their machinists are "old skool" !!!
So I hope I've got it right, it's supposed to be ready to collect next week.

Schrödinger's cat - so which one am I ???

Also bare in mind that pistons are not cylindrical.

EDIT: Although I think most people here probably knew that anyway.

Edited by Rob H on 31st Jan, 2008.

Oh, and I've told them to offset the bores by 0.25mm (0.010") outwards to reduce the risk of getting back a scrap block.
Vizard says "must" for 74mm, "good idea" for 73.5mm but doesn't mention 73.0mm, but when I measured the casting between current bores, I worked out the 0.25mm would not only give more uniform spacings on 73.0mm, but also place the gasket (the one I measured) better.
Again, I hope I'm right !!!

Schrödinger's cat - so which one am I ???

Schrödinger's cat - so which one am I ???