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Engine basics => Engine set up tips => Topic started by: Kim Mortimore on February 02, 2009, 05:10:39 PM

Title: Non-Flat Glow Plug Washers
Post by: Kim Mortimore on February 02, 2009, 05:10:39 PM

I notice the washers that come with Thunderbolt plugs are not flat, but are concave on one side and convex on the other.  The question (possibly dumb) is:  which side is up?

Thanks,
Kim Mortimore
Title: Re: Non-Flat Glow Plug Washers
Post by: Jim Oliver on February 02, 2009, 05:32:15 PM
Kim,

I put the concave side down, thinking that any "crush" will flatten the washer and achieve a better seal than if the concave side is up.  Maybe yes, maybe no?? n~

Cheers,
Jim
Title: Re: Non-Flat Glow Plug Washers
Post by: Ralph Wenzel (d) on February 03, 2009, 05:54:06 AM
What you describe , Kim, is a Belleville washer; a type of lock washer which applies even pressure over its entire area. See:

http://en.wikipedia.org/wiki/Belleville_washer


Title: Re: Non-Flat Glow Plug Washers
Post by: PeteBergstrom on February 03, 2009, 08:41:16 AM
I would suggest that if you are always using the same type of plug that you use the old washer that was on the engine (and providing a good seal) when you replace your glow plug.  Using the threads in the aluminum head to flatten the washer can/will cause premature stripping of the head. (I learned this the hard way when flying F-1 and changing the plugs every flight HB~>)

 If you already have a washer that works, why not keep it in service.

Pete
Title: Re: Non-Flat Glow Plug Washers
Post by: Jim Oliver on February 03, 2009, 10:21:09 AM
Reusing the proven good washer might be OK if the seating area on the new plug is the same as the old plug to which the old washer has been pressure form fitted.  I have actually heard compression leakage after replacing plugs and using the new washer.  The washers appear to be just punched out of copper sheet stock and usually have burrs around the edges which should be smoothed before installation to help insure a good seal with out the need to be over tightened.  When changing plugs in full size aircraft, the compression washers are always replaced, as is the washer used to seal the oil drain plug.  However, these are NOT simply punched out of copper sheet.

The Belleville washer previously mentioned is for an entirely different application and is made of different material---not soft copper as are plug washers.

I have recently begun using a 1/4-32 die to clean up the threads on glow plugs before they are installed; some glow plug threads are slightly damaged/deformed and can damage threads in the relatively soft head material.

I have noticed that most model engines no longer are supplied with brass or other harder material cast into the head and threaded to receive the glow plugs-------now just the relatively soft aluminum head material.

Edited to add:  Installing the concave side down may not be correct in any/all cases.  Some plugs seem to have a slight taper or radius at the area of the plug body that will contact the washer.  In that case, it may be better to install the washer concave up to better fit the washer to the radius............

More than you wanted to know............ ;D

Jim
Title: Re: Non-Flat Glow Plug Washers
Post by: Alan Hahn on February 03, 2009, 11:06:53 AM
My "guess" is that the non-flatness is just coming from some type of punching process used in the production----but I don't know for sure. Of course I think you want these washes dead-soft to make your seal.

Any glow plug experts are welcome to correct me!
Title: Re: Non-Flat Glow Plug Washers
Post by: Jim Oliver on February 03, 2009, 11:24:27 AM
Alan,

I agree with your "guess" about the shape of the washers------just a characteristic of the punchout process.  Some don't seem as concave/convex as others. 

In a perfect world, dead soft, flat and no burrs would probably be just right.....

Jim
Title: Re: Non-Flat Glow Plug Washers
Post by: Ward Van Duzer on February 04, 2009, 09:38:56 AM
Fact or Fiction?

Dunno, but here's what I've always been tol'...

Take a good look at the threaded portion of your plug. The gasket surface is not flat. It is, in fact, beveled. When you put the gasket on, (if it's not already on) the cupped surface of the gasket mates with the beveled surface of the plug.

Always sounded good to me....

Ward-O