stunthanger.com
Engine basics => Engine set up tips => Topic started by: Bill Hummel on June 07, 2020, 05:19:22 PM
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Hi gents, can one of you tell me the thickness of the original stock head shim? Couple engines I have are all different. Like to get them straightened out. Thanks for your help!
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The original head gaskets for the old ST were made of soft aluminum, with a large amount of burrs on the perimeter.
A used gasket measures (without burrs) 0.45 mm.
Probably before being squeezed between the head and the cylinder it could have been something more, I will try if I have somewhere a NEW original gasket.
Massimo
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Two different numbers?
Any change that ST used different shims, to correct for the 6 dimensional tolerances that stack up to determine final head clearance?
( head depth, liner flange thickness, top of piston to wrist pin distance, rod length, top of case to crank centre line, crank stroke)
If you have one that runs well, then use whatever shim is required to get the same head clearance on the others.
Pat MacKenzie
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Two different numbers?
Any change that ST used different shims, to correct for the 6 dimensional tolerances that stack up to determine final head clearance?
( head depth, liner flange thickness, top of piston to wrist pin distance, rod length, top of case to crank centre line, crank stroke)
If you have one that runs well, then use whatever shim is required to get the same head clearance on the others.
Pat MacKenzie
I have used ST engines since the second half of the 70s, I have no memory of two different thicknesses but I cannot rule out that before that time something different did not exist.
Also in the SuperTigre catalog (late 70s - early 80s) the gasket is one with only one ID number (the head gasket was sold with the crankcase gasket as a set).
It may be that M-M has confused with the measures in millimeters or has confused aftermarket gaskets with the original ST ones, these are things that can happen especially when the spare parts accumulate in the drawers and are no longer in the original packaging.
Massimo
Typed after:
At the time the gaskets of different thicknesses were handmade.
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The original head gaskets for the old ST were made of soft aluminum, with a large amount of burrs on the perimeter.
A used gasket measures (without burrs) 0.45 mm.
Probably before being squeezed between the head and the cylinder it could have been something more, I will try if I have somewhere a NEW original gasket.
I would caution everyone that while the stock gasket is fine, you *don't want to go any thinner*, at least for stunt. You can put as much nitro as you want in the stock engine, but even a tiny reduction in the clearance can make put it over the top. The indications are kickbacks when starting, flaming out inexplicably, kicking the prop off (on the ground or in flight), or generally bad running. They were not building stunt engines, they were building the most powerful RC engine they could, so they ran the head clearance as low as they could safely get it.
That doesn't mean that you need to go out and grab a bunch of head gaskets and shove them in there to "soften it up". If it's exhibiting nasty run characteristics like flaming out, then .005, or a replacement stock gasket, is what you need, not .015-.020-.030-1/16" is not better. All that does is kill the power, makes the break weak (or backwards...) and lazy. If you think I am kidding about the 1/16", I have seen that or more, including one genius running .070 or so, with a "hemi", where it would break into a weak two, the airplane slowed down, and it would never recover.
It's the year 2020, you want every iota of power you can possibly get out of it - just like 1984, except that now, you aren't worried about guys with 45FSRs and ST60s, you are worried about guys with PA75s and Plettenburgs.
Small adjustments (.003-.005 at a time), once everything is running properly and you need a tweak, it of course fine, and part of the art of running it. When in doubt, or you get way off in the weeds (which I have, many times) however, put it back to stock and start over.
The ST46 and 60 are among the *very very few* common stunt engines where it is realistically possible to go over the top on compression, when in doubt, leave it alone.
Brett
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...... When in doubt, or you get way off in the weeds (which I have, many times) however, put it back to stock and start over.
The ST46 and 60 are among the *very very few* common stunt engines where it is realistically possible to go over the top on compression, when in doubt, leave it alone.
Brett
I believe this is the reason for Bill's question
Massimo