End gap issue w/ new ring in fresh overbore
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From: Madison, WI, US
End gap issue w/ new ring in fresh overbore
I am in the process of rebuilding a B18A. The block I received had some moderate scoring so I had the local machine shop overbore it to 82mm. I bought a new set of NPR pistons and rings (+1.00mm) oversized to match the overbore. Anyways yesterday I went to set the end gap of the new rings and found that the rings (right out of the sealed package) had a gap larger than the specs listen in the B18a repair manual. The manual listed a gap of .010-.015. in for the top ring. The new rings I have all had a gap of .020-.021 in. Specs for the 2nd ring were .012-.018. The rings I bought have a gap of .018-.020. I am wondering if anyone has an idea of the problem here. Do oversized pistons require a larger gap? Are the rings bady made? Any ideas would be appreciated
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From: strathcedar, nsw, australia
Re: End gap issue w/ new ring in fresh overbore (mvp100)
My understanding is that theoretically, the larger the bore the larger the gap should be. This is because as the ring expands the gap closes, and a larger ring will expand more, closing the gap more. I'd be surprised if the degree to which you've overbored would have a significant affect though (I could be wrong). If the gap closes completely when the ring expands the ring can get forced very hard against the bore wall and cause very serious problems.
In any case, it makes almost zero difference to compression etc if the gap is on the larger side. Even with a large gap the space through which compressed and expanding gasses would have to pass in a very brief period of time is just too small to have a significant affect on pressure loss. Keep in mind that the 'hole' through which gasses have to pass isn't as deep as the ring depth, it's only that part of the gap that protrudes outside the ring groove, i.e it's area is tiny, even when oversize.
When a ring loses compression it does so because it's lost ring tension (usually due to overheating), or it's not being forced outward by compressive or combustion pressures acting behind the ring (because of deposits blocking the path through which this pressure passes into the area behind the ring), or it's become stuck in the groove (due to gummy deposits). In all these cases the ring will lose significant pressure around it's entire circumference, not at the gap.
The gap is often blamed because wide gaps are often found on engines that are losing compression, but this is merely guilt by association. Many engines will lose compression even if the gaps are within spec, and others will retain good compression with overly wide gaps (i.e. outside of spec).
I've read an article (somewhere or other), describing experiments with ring gaps where a given engine was tested for compression and power with ever increasing ring gaps (starting at the min spec gap). From memory it wasn't until the gap was something like 4 or 5 times larger than then the widest recommended gap that either compression or power started to be significantly affected...
In any case, it makes almost zero difference to compression etc if the gap is on the larger side. Even with a large gap the space through which compressed and expanding gasses would have to pass in a very brief period of time is just too small to have a significant affect on pressure loss. Keep in mind that the 'hole' through which gasses have to pass isn't as deep as the ring depth, it's only that part of the gap that protrudes outside the ring groove, i.e it's area is tiny, even when oversize.
When a ring loses compression it does so because it's lost ring tension (usually due to overheating), or it's not being forced outward by compressive or combustion pressures acting behind the ring (because of deposits blocking the path through which this pressure passes into the area behind the ring), or it's become stuck in the groove (due to gummy deposits). In all these cases the ring will lose significant pressure around it's entire circumference, not at the gap.
The gap is often blamed because wide gaps are often found on engines that are losing compression, but this is merely guilt by association. Many engines will lose compression even if the gaps are within spec, and others will retain good compression with overly wide gaps (i.e. outside of spec).
I've read an article (somewhere or other), describing experiments with ring gaps where a given engine was tested for compression and power with ever increasing ring gaps (starting at the min spec gap). From memory it wasn't until the gap was something like 4 or 5 times larger than then the widest recommended gap that either compression or power started to be significantly affected...
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