Aluminum head spacer for '72 combat (2018)

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Deets55 said:
I would also contact Cometic for a base gasket. I know they make them so you might be able to get the exact thickness you need.
Pete
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On my 1360 I doubled up standard Cometic .043" base gaskets to get the required squish clearance.
Perfect sealing, gasket and spacer all in one.
Because it is rubber coated, and in my case there are four rubberized surfaces, the gaskets did compress a bit more than a straight metal spacer, so this has to be taken into account.
.086" torqued down to about .075"
On retorque the change was minimal, didn't even require tappet adjustment.
As Pete suggests, a single rubberized base gasket with core thickness at your spec is likely something Cometic would take care of.

Glen
 
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ILLF8ED said:
I already have the aluminum head spacer. My thoughts on using the cylinder base spacer is it alters the normal throw interface between the lifters and the cylinder. Thanks for responding, but it didn't answer my question about to anneal or not.
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Current advice is to anneal copper head gaskets no more than 20 minutes before use and anneal every time....

As for your 'normal throw interface'? Funny no one else ever worried about that!! The standard followers hang enough out of the tunnels to even contact cams with ground base circles.....and if you think you will measure any angular difference in valve opening closing, I would suggest the manufacturing differences between cams is probably greater than anything you might measure.

Base spacer is the accepted method to reduce compression and has been since....well...forever really!
 
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ILLF8ED said:
I looked up and as Hobot says aluminum is much more malleable than copper. OK Al head spacer is out. Back to copper head gasket, maybe stack two of them. I’m not thrilled with the idea of a cylinder base spacer, not to mention it requires more disassembly.
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JSM will do you a thicker gasket on request....

jsmotorsport.com

Custom Thickness 750-1000 Copper Head Gasket (add pliobond sealant - not included)

Available in various thicknesses to alter compression. 3/32″ and 1/8″ gaskets are available on special order at double the cost (contact us). Crushable thin .005″ copper wire is provided for looping…
jsmotorsport.com
 
SteveA said:
JSM will do you a thicker gasket on request....

jsmotorsport.com

Custom Thickness 750-1000 Copper Head Gasket (add pliobond sealant - not included)

Available in various thicknesses to alter compression. 3/32″ and 1/8″ gaskets are available on special order at double the cost (contact us). Crushable thin .005″ copper wire is provided for looping…
jsmotorsport.com
Click to expand...
But.... he does base gaskets in copper too, and runs an Atlas....Maybe you should chat with Jim?

jsmotorsport.com

Custom 750- 920 Copper Base gasket

021″ thick (more info).
jsmotorsport.com
 
worntorn said:
On my 1360 I doubled up standard Cometic .043" base gaskets to get the required squish clearance.
Perfect sealing, gasket and spacer all in one.
Because it is rubber coated, and in my case there are four rubberized surfaces, the gaskets did compress a bit more than a straight metal spacer, so this has to be taken into account.
.086" torqued down to about .075"
On retorque the change was minimal, didn't even require tappet adjustment.
As Pete suggests, a single rubberized base gasket with core thickness at your spec is likely something Cometic would take care of.

Glen
Click to expand...
Hi Glen

This string is almost four years old for my combat engine. I ended up not using the aluminum head spacer. I used two copper head gaskets. Have since gotten some pushrod tubing to make a replacement shortened intake one that broke. I’m back to one copper head gasket. Compression on both cylinders is 180psi as it was before (130psi with two head gaskets.)
 
You broke an alloy pushrod? If you decide to use a base gasket donot subsequently remove it wear might ensure the top ring hits the bore . Wonder if your head has been over skimmed?
 
oldmikew said:
You broke an alloy pushrod? If you decide to use a base gasket donot subsequently remove it wear might ensure the top ring hits the bore . Wonder if your head has been over skimmed?
Click to expand...
Nope and nope. I broke a chrome moly shortened pushrod. The head has not been “skimmed”. Combat engines came with standard length pushrods…alloy. NV did not compensate for .040” removed on the head and the rockers ended up in configuration that wears the valve guides more than normal. The cure is shortened pushrods to restore a proper rocker geometry.
 
ILLF8ED said:
Nope and nope. I broke a chrome moly shortened pushrod. The head has not been “skimmed”. Combat engines came with standard length pushrods…alloy. NV did not compensate for .040” removed on the head and the rockers ended up in configuration that wears the valve guides more than normal. The cure is shortened pushrods to restore a proper rocker geometry.
Click to expand...
Hi everyone,quite interested in this thread. My engine builder just opened my 1970 750 motor,sitting in my in garage for the last 20 years from the US of A. He found domed pistons +10,probably from Baxter Cycle with an alloy combat style head gasket. The pistons protrude well above the barrels.
My engine builder is confused as to what the previous builder was trying to achieve. Fitting aftermarket high compression pistons then knocking it back with aluminium head gasket, on the other hand it needed something to fill the gap!
Everything is in good condition so are hoping to rering and adjust compression with a base spacer.
Cheers Paul.
 
Macca70cmdo said:
Hi everyone,quite interested in this thread. My engine builder just opened my 1970 750 motor,sitting in my in garage for the last 20 years from the US of A. He found domed pistons +10,probably from Baxter Cycle with an alloy combat style head gasket. The pistons protrude well above the barrels.
My engine builder is confused as to what the previous builder was trying to achieve. Fitting aftermarket high compression pistons then knocking it back with aluminium head gasket, on the other hand it needed something to fill the gap!
Everything is in good condition so are hoping to rering and adjust compression with a base spacer.
Cheers Paul.
Click to expand...
Combat engines did not have a specific “combat” head gasket. They came with a standard solid copper one. The alloy one I mention at the beginning of this thread was possibly an aftermarket intended to restore the .040 milled on a “C” stamped combat head. If your ‘70 has a combat head you likely do not want domed high compression pistons. Do a web search for PowerMax pistons. When I bought my ‘73 750 it had these and they have a slight dome. In any case the group here warned against using an alloy head gasket….they continue to squish out.
 
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ILLF8ED said:
snipped my question is regarding annealing aluminum in this application. snipped
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David, regardless of if the head gasket is made of copper or aluminum, during manufacture, all the holes are made by broaching. Even when using annealed metal, the broaching process will work harden the metal along the edges of all those holes. This is why annealing is recommended. One thicker copper head gasket would be preferrable.
 
The factory did for a time use a thick aluminium head gasket on combats to reduce compression, the gasket was so soft you had a hard time constantly torquing the head fasteners and they were dropped. Covered in NOC notes with a recommendation to put in bin.
 
Jim Schmidt sells copper base gaskets at 0.021”.

And copper head gaskets at varying thicknesses from 0.003” to 0.062” and special order head gaskets at 3/32“ and 1/8“ thickness.

So it should be possible to shuffle things around to get the CR exactly where you want it and you don’t need no soft squashin’ alloy spacer gaskets !
 
What where they thinking when they built and de tuned the combat engine. We have to blame the bean counters.
 
Fast Eddie said:
Jim Schmidt sells copper base gaskets at 0.021”.

And copper head gaskets at varying thicknesses from 0.003” to 0.062” and special order head gaskets at 3/32“ and 1/8“ thickness.

So it should be possible to shuffle things around to get the CR exactly where you want it and you don’t need no soft squashin’ alloy spacer gaskets !
Click to expand...
Thanks all for the information. There is no way the ally gasket is going back on but it seems the PO had to fit it to make these pistons work. Makes more sense to find the clearance by spacing the barrels. Everything has still to be measured and checked.. Hi to Fast Eddie and Dominator,I’m in Lincolnshire and met you both at Cadwell.
 
Macca70cmdo said:
Thanks all for the information. There is no way the ally gasket is going back on but it seems the PO had to fit it to make these pistons work. Makes more sense to find the clearance by spacing the barrels. Everything has still to be measured and checked.. Hi to Fast Eddie and Dominator,I’m in Lincolnshire and met you both at Cadwell.
Click to expand...
Hi back atcha!

I was a bit stressed out at Cadwell cos of my pissing (literally) carbs!

Here‘s some links to the JS gaskets… caution required though… perusing Jim’s website can be bad for your wealth…

jsmotorsport.com

Custom 750- 920 Copper Base gasket

021″ thick (more info).
jsmotorsport.com

jsmotorsport.com

Custom Thickness 750-1000 Copper Head Gasket (add pliobond sealant - not included)

Available in various thicknesses to alter compression. 3/32″ and 1/8″ gaskets are available on special order at double the cost (contact us). Crushable thin .005″ copper wire is provided for looping…
jsmotorsport.com

jsmotorsport.com

High compression .003″ thin head gasket ring set (add pliobond sealant).

Comes with .005″ thin crushable copper wire for eliminating oil leaks at pushrod tunnels and oil return hole, making this a reliable and trouble free setup. This is the easiest way to raise…
jsmotorsport.com
 
ozcruiser said:
What where they thinking when they built and de tuned the combat engine. We have to blame the bean counters.
Click to expand...
Kinda obvious what they were thinking when they built it… wise or not.

The de tuning via silly spacer was probably to facilitate de tuning of existing bikes in the field and perhaps also to allow use of large batches of pre machined heads ?
 
ILLF8ED said:
Combat engines did not have a specific “combat” head gasket. They came with a standard solid copper one. The alloy one I mention at the beginning of this thread was possibly an aftermarket intended to restore the .040 milled on a “C” stamped combat head. If your ‘70 has a combat head you likely do not want domed high compression pistons. Do a web search for PowerMax pistons. When I bought my ‘73 750 it had these and they have a slight dome. In any case the group here warned against using an alloy head gasket….they continue to squish out.
Click to expand...
You don't want domed head pistons in a combat with today's fuel
I did this with mine and I couldn't stop it pinking
I gave up in the end and went to stock
 
No mention so far of this '1970' model having a Combat head, only domed pistons that protrude above the barrel gasket face and I believe is normal for the standard 750 pistons.

www.accessnorton.com

Replace pistons or not?

Re: Replce pistons or not? General rule of thumb for ring gaps is 0.003" per inch of bore, so 0.009" for a 3" bore
www.accessnorton.com www.accessnorton.com
www.accessnorton.com

Piston Deck Height

I’ve been having problems with people commenting on my pictures that things don’t look right. I think part of it’s that I’m using dramatic lighting at angles people aren’t use to seeing their bikes at. The other parts is that I’m messing up. I haven’t measured it yet but I think spec is .050”...
www.accessnorton.com www.accessnorton.com
 
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I tried alumin
kommando said:
You don't want it soft or it will spread like cheese as the factory ones did.
Click to expand...
I tried aluminum header flange gaskets once because I could not find copper on short notice.. They too spread like cheese. Went back to copper and all was well again.
 
Macca70cmd.
Do your domed pistons look like these. This is what was in my ‘730 750 when I bought it in 1981.
Aluminum head spacer for '72 combat (2018)
 
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