Roundness cylinder (ovalization bore) 1098cc

[MMDream]

Hello,

I'm looking for roundness tolerance and conicity on 1098cc. (I've 0.02 mm on mine)
I just know the original size: 2.543'' (64.58mm) and the clearance between piston and bore:
Top of skirt: 0.0021'' to 0.0037'' (0.025 to 0.070mm)
Bottom of skirt: 0.0005"" to 0.0011'' (0.013 to 0.028 mm)

In mesure the bore: 64.60 to 64.62mm
+ 0.02 mm compare to original

My pistons bottom of skirt: 64.54 mm

Then I have 0.04 to 0.08 mm of clearance at bottom of skirt
and 0.02 mm of ovalisation


Sorry for my English.

Can you confirm the good word for ovalization??

[bmcecosse]

Looks like absolute minimimal wear to me - what's the problem?

[philthehill]

Unless you have specialised bore and piston measuring tools I would be wary of assuming any difference in measurements at various points for both the piston and bores.

This is the type of tool you require for accurate measurement of cylinder bores:-

http://www.ebay.co.uk/itm/mitutoyo-bore ... Swa-dWknUU .

This is the type of tool you require for accurate measurement of the piston and piston skirt:-

http://www.ebay.co.uk/itm/MITUTOYO-103- ... SwL7VWsR8L

Whilst pistons look round they are in fact slightly oval.

[MMDream]

OK thank you for the links.

I Have measurement tools.
In fact I want to know the maximum ovalization of bore for 64,58mm diameter (0.01 - 0.02 mm...?)

I found 64,62 mm maxi for original bore 64,58 (0,04 mm more than original)

Somebody can confirm the clearance of skirt:

Its whrite on manual :
Top of skirt: 0.0021'' to 0.0037'' (0.025 to 0.070mm)
Bottom of skirt: 0.0005"" to 0.0011'' (0.013 to 0.028 mm)

Because I found 0,04 to 0,08 mm of clearance between bottom of skirt and bore.
It's really big compared to data of manual

It's just to be sure I need to replace piston and rebore.

thank

[bmcecosse]

It's not 'really big' - it's absolutely minimal!

[philthehill]

The bores are round so in reality you can ignore the piston skirt clearances as they are only relevant to a BMC freshly bored engine and where the piston is graded to the bore (grade indicated by number in diamond on top of piston). Not all original bores were bored to the exact specification as there had to be machining tolerances so required graded pistons.
Nowadays with the lack of graded pistons the bore should be graded or bored within tolerance to suit the piston.
The most important measurement is the bore dimension. Ovality does not really feature - it is more of a red herring especially as regards worn bores. Either the cylinder bores are good or they are not.
It is poor practice to fit new pistons into worn bores so you have the choice of running with what you have or re-bore.
If you need to have the engine re-bored you will need new pistons and rings to suit the re-bore anyway.
What you need to know is what the wear in the bore is.
The standard bore for a 1098cc engine is 2.543" or 64.58mm.
As an early indication as to whether the engine needs a re-bore - If you run your finger nail up and down the bore and if there is an wear ridge that catches your finger nail it is time fore a re-bore.
Measuring the bore below the top ring wear track (at 90 degrees to the piston wrist pin as that is were most wear take place) and if the bore is 64.58mm (2.543") or more but less than plus 0.254mm (+0.010") wear the engine may require a plus 0.254 (0.010") (or larger re-bore dependent upon measurements found) re-bore. Re-bores are usually determined in 0.010" increments.
You can check for ovality of the bore by measuring in line with the piston wrist pin and comparing that measurement with that measured at 90 degrees to the piston pin will give the ovality of the bore.
It would be useful if you posted the current bore sizes on this thread so as to be able to give a more constructive comment.
Phil

[MMDream]

Thank for your answer

Piston size (at bottom skirt and perpendicular to shaft):
64.54 mm

Bore size are 64.62 and 64.60 (measurement perpendicular)

Then +0.04 mm for the maxime bore size

[bmcecosse]

Which is a tiny amount of 'wear' - either use as-is , or glaze bust and fit new rings.

[MMDream]

OK thank alot for your quick responses.

I'll do that. I glaze bore and change piston rings.