Piston Ring End Gap

[mhyphenl 10 1 Cars]

Just re building the engine and got round to piston rings. Haynes says end gap of 0.3 to 0.5 mm, I have 0.8mm! How critical is this? Do I just need new rings? What is involved in re honing, do I need just different rings or new Pistons and liners???

Sorry for the quick fire questions but I've stopped building until I'm sure what to do!

[Tom Fenton 1,542]

Are these used rings and liners?

[Stu 18]

Where are you measuring the gap? At the top of the bore or half way down by any chance?

[mhyphenl 10 1 Cars]

Yes, used Pistons liners and rings. Measured about an inch down so on the worn part of the liner!

[Stu 18]

Used rings you say? Are you not replacing them as part of the build?

[petert 588]

It only takes a small change in bore wear to make a large change in ring gap, because perimeter = pi x D. Thus 0.1mm of bore wear will equate to 0.3mm increase in ring gap. What was probably 0.5 has become 0.8.

[mhyphenl 10 1 Cars]

Hi, yes I get you. I guess my point is, bearing in mind there are probably other areas of the engine that are sub optimal, is an end gap of 0.8mm a significant problem in both running quality and performance? If it is then what's the protocol, just new rings or is it the liners that have worn? In which case are they honed and oversize rings or is it new liners pistons and rings?

[petert 588]

The liners will be worn. If you install new, "pre-gapped" rings, your end gap won't improve. Ideally you need to hone the liners and fit new ungapped rings. However, the excessive gap itself is not the problem. As the bore size increases, the tangential force of the ring on the bore decreases, allowing blow-by. Any new rings will seal better than old rings for this reason.

[mhyphenl 10 1 Cars]

Ok, I take it that in gapped rings need to be fitted professionally? I understand what you mean in that the nearer the rings get to their natural unfitted shape the less force they exert on the liner but is .1 of mm in diameter going to have a significant difference? I'm not really trying to be tight but on the other hand it's going to be a £150 job ish!

[petert 588]

Pre-gapped rings are for muppets, with little to no fitting experience. Ungapped rings are for professional use, where the installer will increase the ring gap using special tools, for the desired application. If money is tight, throw the old rings in and hope for the best!

[Miles 331 1 Cars]

For the cost of new liners I go that route all the time as finding a std liner that is out of tolerance and cannot be honed makes it allot more in the long run

[mhyphenl 10 1 Cars]

So miles, you suggest new liners and rings!? What kind of money are we talking for that?

[Miles 331 1 Cars]

Around £175.00 for liners, £105 for rings