Engine basics > Engine set up tips

tight fitting heads

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Jim Thomerson:
Back when we were racing, we found that if we had a head which fit down into the cylinder tightly, not an interference fit, but nice and sung, that we would get as much as 1000 RPM more as compared with a similar head with a sloppy fit.  I notice in most of my stunt type engines that the head is a sloppy fit.  Has anyone tried making a head or a head button the same as a stock head but with a snug fit in the cylinder?

Bob Reeves:
Understand I have no idea what I am talking about but wouldn't a tight fit just raise the compression?? Guess it could also transfer cylinder heat to the head better.

Tom Dugan:
How sloppy of a fit are you referring to? 

I would not think that you would want a real tight fit, or the metal could expand and possibly distort or crack other parts as it heated while running.  Also, that tighter fitting part would become more easily locked in place with burnt and baked castor which would make it more difficult to work on if needed.  I can see Pros and Cons either way.  I suppose it boils down to the design of the engine and how the designer/modifier intends the engine to work.

Depending on what kind of machinery we are talking about...close tolerences aren't always best.  However, we do tend to equate close tolerance with precision workmanship, like a watch...but watch parts are not rotating at 12000 RPM and building heat from internal combustion.

Just my thoughts.

Thomas

Jim Kraft:
Jim; I don't know how much  difference it would make on a stunt engine, but I don't think it would hurt to have a very snug fit on an ABC or ABN as the sleeve expands more than the head anyway I would think. By the way, a while back you posted  your way of putting a wing in a profile by heating epoxy, and letting it run in to the joint while having the other side taped to keep it from running through. On my last profile, a Magician, I filled the joint with epoxy and applied my heat gun to it, and it ran in like water. I have been flying it for quite a while, and there are no cracks anywhere. Thank you for a very good tip.

rustler:
Our tame engine guru John Dixon has a few theories. With a loose fitting head the space up the side of the plug is effectively wasted space. Gas is squeezed up there, but the burning of it is most inefficient, possibly only completed long after initial firing. Similar reason for increased performance with Cox/Rossi type head-plugs. On conventional plug there is lost volume up the threads. This doesn't exist on the Cox/Rossi set-up. With a loose fit head you always find burnt castor up the side of the plug, so fuel definitely gets up there.
Assuming cylinder thickness is normal, a tight head is unlikely to split a steel cylinder. It can take the strain.

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