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Author Topic: tight fitting heads  (Read 944 times)

Offline Jim Thomerson

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tight fitting heads
« on: October 03, 2006, 02:23:02 PM »
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?
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Offline Bob Reeves

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Re: tight fitting heads
« Reply #1 on: October 03, 2006, 02:30:30 PM »
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.
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Offline Tom Dugan

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Re: tight fitting heads
« Reply #2 on: October 03, 2006, 06:46:53 PM »
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
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Offline Jim Kraft

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Re: tight fitting heads
« Reply #3 on: October 03, 2006, 07:05:09 PM »
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.
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Jim Kraft

Offline rustler

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Re: tight fitting heads
« Reply #4 on: October 04, 2006, 01:13:21 PM »
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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Ian Russell.
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Offline phil c

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Re: tight fitting heads
« Reply #5 on: October 05, 2006, 09:26:44 AM »
This is a real effect.  The thin space between the head and cylinder does more than just stop that fuel from burning.  It actually reduces combustion pressure through the whole cycle.  Same for the exposed threads on the glow plug.  The wedge heads in car motors were developed to do just that.  Quench some of the combustion and prevent preignition.

If you fit the head tight and use a Nelson plug head button you'll probably gain some 1000-1500 rpm.  That is horsepower you don't have to get burning fuel, so gas mileage will go up.  More hp available at all rpm's means you can back off on the rpm.  This will change all your tuning parameters, so be ready for some work.

The head should be tight enough that you can just work it in place, if everything is spotlessly clean.  Make sure you don't get it bound up at an angle putting it on.  That will ruin everything if you don't spot it before adding the bolts.  Icing the head and inserting it into a hot engine will help.
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phil Cartier

Offline L0U CRANE

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Re: tight fitting heads
« Reply #6 on: October 05, 2006, 09:59:34 AM »
Jim, and all -

GMA wrote in several places about the desirability of minimum clearance between the rim of the head in the cylinder. I recall an article in, think t'was, an Aeromodeller Annual, where he described his method of turning down said rim and press-fitting an iron band on it.

The band was precision-turned to make a very tight fit in the cylinder.

That was back in the days before ABC/ABN/AAC... the days of Meehanite, or ringed aluminum, pistons in steel or iron sleeves, occasionally chromed for a ringed piston.

He cited a considerable gain for high-performance engines, and had a reputation for prepping very fine engines for such use.

For the somewhat less demanding conditions we have in stunt, at least a very nice free-running fit could improve cooling. A coat of charred oil is not a good heat conductor. However, I suspect the main heat flow from the head to the casting is via the tight clamping to the top of the cylinder...
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Offline Busby

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Re: tight fitting heads
« Reply #7 on: October 06, 2006, 09:35:30 AM »
I am surprised that no one has stated the benefit of insuring that the liner is forced or helped to stay round and helps the fit while under temperature changes.
Busby
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Marshall Busby

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