Piston Ring Gap

[Gannet]

I am in the process of fitting a replacement block onto a FV and re-using the original piston. The piston ring gap is 0.031 which I think means that new rings are required. Is that correct?

With new rings what should the gap be set to?

Thanks

Jeremy

[Stelios_Rjk]

Great question, I would also like to know what is the correct gap for 64cc and 102cc Seagulls.

[Horsley-Anarak]

Seagull say 0.004" or 0.1mm.

I have never measured the rings, I must start doing that.

H-A

[Stelios_Rjk]

In an older post I found that the gap should be 0.0025"-0.003". This seems to be very low to my understanding (less than 0.1mm). So I assume that 0.1mm should correct.
H-A, did you find the info in a book or?

[Horsley-Anarak]

H-A, did you find the info in a book or?

Yes Seagull Service Sheet No.3.

H-A

[Stelios_Rjk]

That's nice, thanks!

[1650bullet]

I am in the process of fitting a replacement block onto a FV and re-using the original piston. The piston ring gap is 0.031 which I think means that new rings are required. Is that correct?

With new rings what should the gap be set to?

Thanks

Jeremy

Hone the block out and put it all back toghether using the rings you have got now Gannet. My TC-102s rings had a 39 thou gap and ive got a compression of 55psi . Thats fantastic for a 102. I took the barrel of my nieghbours 40 model and honed out the cyl and fitted it all back up with the original rings and the cyl pressure went from 60 to now 80psi just from a good clean and hone. Try what youve got now. Theres only 2 nuts that holds it toghether.

[Gannet]

Thanks for that advice. In terms of sealing, your experience would suggest that bore finish is as important, if not more so, than ring gap. I take note and will investigate honing/glaze busting. I think the theory is that oil is trapped in the small scratches to improve sealing and thus compression.
It made me realise that I have asked the wrong question.
I should have asked:-
Would a 0.031" ring gap reduce the performance by a noticeable or significant amount on an FV?

Theoretically of course a small ring gap is better -provided of course under running conditions there is a gap, but maybe a large gap (ie 0.031) doesn't make any real difference???
Perhaps somebody with knowledge could comment.
Thanks

Jeremy

[1650bullet]

Thanks for that advice. In terms of sealing, your experience would suggest that bore finish is as important, if not more so, than ring gap. I take note and will investigate honing/glaze busting. I think the theory is that oil is trapped in the small scratches to improve sealing and thus compression.
It made me realise that I have asked the wrong question.
I should have asked:-
Would a 0.031" ring gap reduce the performance by a noticeable or significant amount on an FV?

Theoretically of course a small ring gap is better -provided of course under running conditions there is a gap, but maybe a large gap (ie 0.031) doesn't make any real difference???
Perhaps somebody with knowledge could comment.
Thanks

Jeremy

Cant really answer that for you Gannet. Its probably because of all the oil in the fuel mix as well.

[Todd]

Theoretically of course a small ring gap is better -provided of course under running conditions there is a gap, but maybe a large gap (ie 0.031) doesn't make any real difference???

I think you answered your own question, Jeremy. In a high performance, high compression engine the gap becomes more critical. Not so much here.
The .004" spec is more a minimum to insure a newly installed ring has room to expand without putting undue pressure on the cylinder walls. It also provides a baseline for judging wear: If you can assume your engine started life with an end gap of .004" and now has a gap of .031", but it still runs well, then you know that .027" in ring and cylinder wall wear is acceptable.

Todd

[Gannet]

Yes, I agree with you.
However, I should have explained that I haven't had this engine running with it's cracked block, so i do not know how it performed. I also have not measured the ring gap in that block.

I suspect that this will not be the last engine that I am going to rebuild, and so I am trying to learn about piston ring gap. I could of course just buy the rings and fit them (with 0.004"gap), but I would not be any the wiser.
I expect that I am not alone in this situation.
The opposite is probably true - ie lots of Seagull experts will have the experience and know the answer.

Jeremy

[Charles uk]

The 31 thou gap, are you measuring that with the ring in a cylinder?

[Gannet]

Yes.
Squared up with the piston skirt. Everything 'scrubbed' clean. About 3/4" done the bore.
Jeremy

[Rex NZ]

A large ring end gap can have several consequences.

Firstly & most obviously is the loss of compression & combustion pressure

Secondly, is the increased likelihood of the combustion flash blowby igniting the crankcase mixture. This results in the motor making a terrific cough out the intake. In the case of a reeded motor it can damage the reeds

[Stelios_Rjk]

When does the sealing of the rings fail and the fire from the combustion leaves a black mark on the piston under the rings?