over size piston rings

[Rob Frankham]

If fitting std rings in old bores you need to check the gap with the rings as close as you can to the bottom of the bore and gap them accordingly.

Sorry, I have to query that. You need to check the gap in the swept part of the bore. In use, the rings don't get anywhere near the bottom of the bore which sees no wear. The best place to check is the middle of the swept area of the bore... i.e a bit above the middle of the bore... which is normally easily visible.

Rob

[jackonz]

If fitting std rings in old bores you need to check the gap with the rings as close as you can to the bottom of the bore and gap them accordingly.

Sorry, I have to query that. You need to check the gap in the swept part of the bore. In use, the rings don't get anywhere near the bottom of the bore which sees no wear. The best place to check is the middle of the swept area of the bore... i.e a bit above the middle of the bore... which is normally easily visible.

Rob

Sorry Rob but in this case you are mistaken, you have to measure the ring gap at a totally unworn part of the bore and that is down the bottom, that is what was drummed into us when I did my apprenticeship in engine reconditioning and there is a good reason for it, new rings when compressed to the bore they are made to fit will sit correctly in the bore if you do it at the point you are suggesting they will not sit correctly as there will be some wear and you will not be able to check for the correct ring gap.

[Rob Frankham]

I'm afraid we'll have to disagree on that one. Ring gap is a measure of ring AND bore wear so it needs to be done in the area swept by the ring otherwise the primary purpose of the test is lost. If you have a full workshop at your command with an accurate bore gauge it's easy to check for bore wear but for normal mortals, the check posed by measuring the ring gap is important. It's also sight quicker and easier than removing the piston.

To put it another way, the purpose of measuring ring gap is to ensure that compression and blowby aren't affected by gaps that are too large. If you do the check at the bottom of the cylinder, then fit the rings in a worn bore, the gap will be too large causing loss of compression and blowby. How much gap is excessive is a moot point and most engines will run reasonably satisfactorily with gaps somewhat larger than spec but that's rather beside the point.

In this case, working from first principles, I have to say I prefer the teaching I receive rather than yours.

Rob

[billy192]

thanks jackonz
they don't have any R80 pistons. i will keep searching.
billy