piston to bore clearance

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johnny650
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firekdp wrote: Tue Nov 13, 2018 11:57 am That testing was done with new rings, of the same size, with altered ring gaps. So only measuring the affect of the ring gap itself. The sealing pressure of the ring itself would be identical.
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yes of course the sealing pressure would be the same but we were not referring to that other than to state that AE found that blow by is caused by worn rings not ring gaps !....

AE were only trying to measure one factor ...ring gap and the effect on performance and compression.

you never test for 2x things at the same time or you'd never know which aspect tested ,..produced any result so you make sure that all other factors in the tests are identical otherwise the results are worthless.
johnny650
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firekdp wrote: Tue Nov 13, 2018 11:57 am That testing was done with new rings, of the same size, with altered ring gaps. So only measuring the affect of the ring gap itself. The sealing pressure of the ring itself would be identical.
However, we are only measuring the ring gap to determine wear, if the ring gap has got significantly bigger since a rebuild, that is only down to ring or bore wear, so sealing pressure/compression will be diminished.
I think we are maybe talking at cross purposes here firedp. I missed the significance of the second part of your post.

You have made a very important point . If the oversize ring gap is indicative of significant wear of the rings resulting in blow-by then yes there could be a significant loss of compression resulting in a loss of compression . the rings would need to have some huge mileage to get that worn unless the initial ring gap was insufficient and the cylinder and piston was subject to long term over heating

Muppet said that an oversize ring gap would cause 'quote 'Massive ' performance loss.
I was taking issue with that statement because I have been unable to find any reliable reference source anywhere that bears that out
Warkton Tornado No.1
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Piston to bore clearance is quite 'critical' IMHO, though I well recall (many years ago!) Malcolm Anderson struggling to start his Group Four bike, yet it won races with a seven thou' gap. I think I am correct in saying that he was so busy @ MSC, that there was simply no time to strip that engine between events to correct the issue.

As for the issue of ring gap, my experience is that a larger gap than the various recommendations is not so critical, particuarly in the case of OEM port widths & two or three ring pistons.

However, larger port widths &/or an increase in the number of ports, inevitably exasperate wear, as does the lack of filtered carburation & higher engine speeds & the ring(s) material.

My advice is that with any modified engine, you need to keep an eye on the ring gap, because although I doubt it leads to power loss, in the two stroke engine, there are other consequences to consider. The obvious, infamous Honda piston conversion had problems with the ring peg material, & any tendency for the ring gap to increase would allow the brass pegs to wear out completely, causing the wreckage of the top end.

(I hope that is a helpful, diplomatic, input to this debate & is certainly intended as a good natured observation ;) )
johnny650
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very good points WT and very welcome observations.
I have a feeling that Muppet and I may have been talking at cross purposes as it were.

Over-sized ring gaps are usually indicative of worn rings which as Muppet says could have a significant effect on compression and performance .

However in this instance I understood that the rings were recently fitted with just a few thousand miles on them so I perhaps wrongly assumed that the rings could not be worn to any significant degree and any slight enlargement of the ring gaps would not be the cause of poor starting and loss of performance.

My original 64 Li150 engine had seen over 25000 miles over possibly 54 years and although slightly down on compression still started 2x kick and ran fine
johnny650
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Warkton Tornado No.1 wrote: Wed Nov 14, 2018 4:05 pm The obvious, infamous Honda piston conversion had problems with the ring peg material, & any tendency for the ring gap to increase would allow the brass pegs to wear out completely, causing the wreckage of the top end.
incidentally WD No1on this related subject of ring gaps . I was going to post some images of a fairly new, used piston that I received recently where a part of the top ring had broken off and gouged the cylinder wall.

On close examination it became clear that although the PO may have sized the ring gaps correctly,... the piston ring had sat up proud on the ring pin and consequently a small piece of the top ring had caught on a port and broken off.

measuring the piston ring pin height and also the depth of the overlapping part of the piston rings it became clear that the rings were not compatible with the piston .

Who bothers to check the height of their piston ring pins and compares that with the depth of the ring cutouts at the gaps ? ....

Anyway long story short the rings just needed dressing back so that they didn't sit proud of the piston over the ring pins and the barrel ports needed a slight bevel which also hadn't been done by the PO.
I'll post some images when i get a chance as it is a useful tip to engine builders that may not be aware of this potential issue
Warkton Tornado No.1
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Interesting point, & one reason that the type of ring peg that fits half in the groove, the other half above it (I can't think what the correct terminology is as I don't have Bell's book to hand!) is to be preferred IMO.

I avoid the generic Lambretta piston, whenever there is a choice, because even pattern Japanese motorcycle piston & rings are better designed & inherently stronger, as well as a damn sight cheaper.

It's always worth browsing Grampian Motors online pages for geometry of the likely alternatives & they even let me have an account!
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