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Engine basics => Engine set up tips => Topic started by: the original Steve Smith on October 10, 2006, 10:27:43 AM

Title: Head Shims
Post by: the original Steve Smith on October 10, 2006, 10:27:43 AM

More questions cause I don't know.

What is a head shim and what is the purpose?  I am assuming that they are additional head gaskets.

Would adding additional gaskets under the glow plug have a similar effect?

Thanks for any explanations.

Title: Re: Head Shims
Post by: RC Storick on October 10, 2006, 10:13:39 PM

Adding head shims lowers the compression. (Taming the run down) Adding washer to glow plug will have little or no effect.

Title: Re: Head Shims
Post by: Bob Reeves on October 11, 2006, 11:25:45 AM

The first one is a head gasket the next one added is a shim  ;D

Title: Re: Head Shims
Post by: Bill Little on October 11, 2006, 02:08:42 PM

Quote from: Robert  Storick on October 10, 2006, 10:13:39 PM

Adding head shims lowers the compression. (Taming the run down) Adding washer to glow plug will have little or no effect.

I have a feeling that adding washers could have an effect on the engine run.  It won't affect the compression ratio much, but it would cause a change to the glow plug's *effective* heat range.  Getting the element further away from combustion will cool the plug, like running a short reach plug in a long reach plug engine.

Bill <><

Title: Re: Head Shims
Post by: Joey Mathison 9806 on October 11, 2006, 08:06:33 PM

plug depth is very critical at least on speed engines

Title: Re: Head Shims
Post by: Dennis Moritz on October 12, 2006, 03:31:07 AM

Adding plug washers may cause flame out. Could reduce the ignition heat. Don't want to do that the way we fly. Hard enough to keep the engines lit, doing tight fast maneuvers. Head shims are an easy way to tame some beasty motors including FPs and Magnums, LAs sometimes benefit from a head shim. Actually serious stunt iron like PAs and Ro-jetts are often tuned with shims to match prop, fuel and break.

Title: Re: Head Shims
Post by: Tom Perry on October 12, 2006, 07:02:57 AM

I've heard that when the LA40/46 is purchased as a control line engine it has an extra head shim.  Can anyone verify if this is a fact or not.

 ???

Title: Re: Head Shims
Post by: Joe Messinger on October 13, 2006, 08:06:54 PM

Tom,

I just had the head off a new LA .46 and there was no head gasket (s) there.

Joe

Title: Re: Head Shims
Post by: Crist Rigotti on October 13, 2006, 08:28:16 PM

Joe,
Yeah there's one in there.  It's in the head groove where the cylinder fits.  Gently take a #11 blade and pry it up by one of the screw holes.

Title: Re: Head Shims
Post by: Joe Messinger on October 18, 2006, 08:44:02 AM

Crist,

Thanks.  Obviously, I didn't look close enough.

Do all of the OS LA engines have the same head gasket set up as the.46? 

Joe

Title: Re: Head Shims
Post by: L0U CRANE on October 18, 2006, 07:42:07 PM

Steve,

Much worth thought here already.

Most modern engines I've been into have a modified "squish band and bowl" type combustion chamber. This allows added gaskets to adjust compression ratio better than the original 'squish/bowl' layout, which was usually a wide, flat band with a round bowl.

That had definite advantages for ultimate power applications, but adding gaskets -shims, if you will - corrupted the effect of squooshing all the burnable fuel/air mix into the 'bowl' at the critical moment.

The recent approach makes it easier for us to play with compression ratio. Certain engines benefit from additional tailoring of the 'squish' shape, if done right by the right hands.

So, all this as just background, yes - adding a gasket or two under the head adjusts compression ratio nicely. Results are usually a more easily achieved 4/2 run, although the engines' design factors seem to urge using the low, richer 2-cycle mode. Trying to drag RPM down to the 4/2 run zone usually forfeits a lot of the power we could get, useably, from the same engine. Power rises with RPM, until the torque curve falls so far that power drops, too. (HP roughly involves torque times RPM, with some additional fiddling factors.)