piston ring size - confused

I have spent some time trying to work out what size replacement rings to get - the (unmarked) existing rings in the car have gaps way out of tolerance. I don't have a tool to measure the bore width precisely, and all my calculations using measurements of the existing rings led to nonsense. So I gave up, spent a few £ and ordered some 010 o/s piston rings: that way I could work out precisely what my bores needed (e.g. 010 might be right, if not, i could work out what size I needed).

So the rings arrived today. I do have a bit of a lip at the top of the bore, so i took one of the compression rings, put in down sideways and then tried to level it out. But it was too big! As it was getting close to being level, the ends met i.e. no gap.

This cannot be right. Even if my bores had no wear (which the lip seems to contradict), I should surely be able to get 010 o/s rings in (as standard rings should have a gap of 015 - 020).

I have double checked - on the website it says rings are for series 2 and series 3, petrol 2.25. The part number on the britpart box matches what i ordered.

I feel i must be missing something obvious- any ideas?

You can grind the end of the rings to get the correct gap, assuming they are flat ended.
You want an unworn part of the bore for reasonably accurate numbers.

The numbers do not work as you think, a small increase in bore dia will see quite a bit larger ring to fill the bore.
Cylinder/circles are funny things when it comes to maths.

Are there any numbers stamped on the piston I.E STD/ +010 .

Is the block out of the car?
Because really if you have a lip it’s worn.

If you can’t get a 010 to be flat I would guess it’s standard/worn not enough for +10. But then I guess you know that now.

What are you trying to achieve?

J

>>I feel i must be missing something obvious- any ideas?

I suspect the missing ingredient in your thinking is a factor of pi linking the diamter and the circumference.

The oversize 0.010" piston ring will be 0.0314" longer than the standard one, not 0.010"

Tall Ratbag said:

>>I feel i must be missing something obvious- any ideas?

I suspect the missing ingredient in your thinking is a factor of pi linking the diamter and the circumference.

The oversize 0.010" piston ring will be 0.0314" longer than the standard one, not 0.010"

Click to expand...

true.

below is a photo of the piston head. I know the X marks the front of the engine, but i don't know what any of the other markings mean i.e. 1) a Capital "A" in the centre 2) a "1" (this piston does come from bore 1...) 3) a small logo with "RC" and either "32" or "92".

I suspect the next step is to buy 1 pots-worth of standard size rings and see how they fit.

Have you tried fitting them in the top of the bore flat, they will be smaller in dia than your cackhanded way

I can't fit them at the top - they are too big. But as there is a lip at the top, i couldn't deduce whether the rings were too big or the bore had worn. Hence the need to try them in the middle of the bore.
Anyway, i will get standard ones and repeat the experiment

I am still a bit confused as to what you are trying to do?

If its worn that set of rings/bore and has a lip, surely its time for it to be measured correctly and rebored?

J