Bore and piston wear

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Pete-B

Member
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44
Location
Gloucestershire
I'm currently doing by first engine rebuild on a 3.9 with an interim block. I though I'd check for bore wear by inserting the pistons upside down and seeing if there was any sideways movement. There was a small amount in one direction but a noticeable amount more in the other direction which I assumed meant the bores were slightly oval. But if I turn the piston around 90 degrees the extra movement moves with it suggesting the pistons are slightly oval. The greatest movement is in the same direction as the gudgeon pin.

Measuring the pistons seems to indicate they're perfectly round and I can still see the lines in the piston casting that go all the way round, again suggesting they're not worn.

The rebuild manual says the pistons should be 0.018 to 0.033mm smaller than the cylinder bores so I thought there should be virtually no movement.

Has anyone else found the same problem? Is it likely to be down to the bores?

Thanks
 
Measure the bores with internal micrometer, is there a ridge where the top ring stops at tdc, also insert a piston ring, push in with the piston, to various depths, and measure the ring gap
 
Pete-B said:
I'm currently doing by first engine rebuild on a 3.9 with an interim block. I though I'd check for bore wear by inserting the pistons upside down and seeing if there was any sideways movement. There was a small amount in one direction but a noticeable amount more in the other direction which I assumed meant the bores were slightly oval. But if I turn the piston around 90 degrees the extra movement moves with it suggesting the pistons are slightly oval. The greatest movement is in the same direction as the gudgeon pin.

Measuring the pistons seems to indicate they're perfectly round and I can still see the lines in the piston casting that go all the way round, again suggesting they're not worn.

The rebuild manual says the pistons should be 0.018 to 0.033mm smaller than the cylinder bores so I thought there should be virtually no movement.

Has anyone else found the same problem? Is it likely to be down to the bores?

Thanks
Click to expand...
pistons are oval till they get hot,which is why a piston is measured at 90 degrees to the gudgeon pin at bottom of skirt as this is the largest diameter when cold, standard pistons vary in size and are matched to make set of 8 but can vary a thou or two compared to another set of 8 likewise with blocks,blocks have a letter stamp as do the pistons so they can be matched when they were built in the factory ,if you rebore an engine you bore to the piston actual size plus clearance
 
Yes there is a ridge and if I put the rings in the bores the gap between the ends is higher than it should be so I guess that means I'll need a rebore or top hat liners.

Glad to hear that pistons are oval when cold, explains what I was finding and also means I should be able to re-use my current ones as they don't look worn.

Thanks for your help.
 
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