ring end gap...331

[simonjrwinter]

I'm just about to start building up my 331 stroker engine. I have decided to run a fairly low compression ratio as I may want to add a supercharger in the future (when I can afford it)
What end gaps should I have bearing that in mind? Its bored over size by 30 thou.
Thanks

[speed220mph]

The rule of thumb is 0.004 in. for each inch of bore diameter. So figure a 0.016-in. minimum gap for a 4.00-inch bore. Testing has shown no compression loss or increase in blowby if you double that figure, but I wouldn't do that. But to ensure the ring ends don't butt, use a 0.020-inch ring-end gap.

I hope that helps.

[wilf leek]

Simon - not sure what pistons you are using, but there is a useful table on the K Black piston site at http://www.kb-silvolite.com/speclear.htm#CHART

It gives ring gap recommendations for a variety of applications, but bear in mind they are for hyper-eutectic pistons, not forged ones.

Tom - out of interest, would the type of piston used (forged, hyper, plain cast) affect the ring gap requirement, or is this independant of piston expansion characteristics?
BTW - a great pleasure to meet you in London Oh.

[speed220mph]

Hi Wilf: Great to meet you, too. Hope the good old USA treated you well. And, yes, most Cobras used in the colonies are fitted with Ford engines. Want to keep them "real" don't we.

About ring end gap, it's the top ring we're mostly interested in since it's the one that gets the heat, thus the most expansion. As for piston material, doesn't much matter because the fire is against the ring. I suppose rings used a forged piston may run slightly cooler because of the higher heat transfer of the forged piston, I wouldn't think it would be much. What does matter is how hot combustion gets, thus the bigger gaps with blown engines that are under load for a long time such as boats, thus the 0.035--0.050 in. gaps. For normally aspirated "short blast" engines, gaps are smaller because running temps are less. The object is to end up with the same running gaps without the end gaps butting.

Again, I'd go with the minimum 0.004-in. per inch of bore, but go to 0.050 in. per inch minimum to be safe with a street engine that will see an occassional blast. For road racing, increase this to 0.060 in. per inch of bore. It's better to err on the high side here because there's virtually no blowby difference until gaps are huge. On the other hand, things get ugly in the bore if the ring ends butt.

I hope all things went well for you at DVSF III and you'll be back again.

[wilf leek]

Tom - thanks, you confirmed what I suspected about the ring gap - it's just about the ring and combustion temps, not the piston material.

Yup - the US treated me fine, I enjoyed myself immensely, even though I sadly associate with Gasholes.

That was my second 'Fling, pretty sure it won't be my last.
One day I might even get my own car over. In the meantime I continue to apologise for all the Ch**y engines my countrymen fit into their Cobras.

[trularin]

.018 and .020.

Especially if you are running hot.

Just my $0.02 worth.