Aluminum head spacer for '72 combat (2018)

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ILLF8ED

ILLF8ED

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I will be replacing the copper head gasket on my combat 750 with a thicker aluminum spacer shaped the same as a head gasket. Several reasons for this, but my question is regarding annealing aluminum in this application. Is it necessary and I see on line bring up the material to 775 degrees then slowly cool. I'm not inclined to go through this process. My plan is to use the spacer which by the way has been previously used and put silk around the cylinder bores, pushrod holes and one oil passage. Comments from anyone that has experience using these aluminum spacers to lower compression?
 
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
 
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.
 
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
Click to expand...

I did a Google search about annealing aluminum and quickly learned that the annealing process depends on what the alloy is, and that some alloys can be annealed and some cannot. One technique was to cover the aluminum with acetylene soot and heat the part until the soot burns off. So, the best answer to your question I could find is, maybe.

I used to anneal copper head gaskets with a propane or MAP gas torch, but realized that achieving a consistent heat soak, without an oven, would be iffy at best. The short read I did on annealing aluminum convinced me that annealing copper was easy by contrast.

If you know the alloy then Google is your friend, not to say that achieving a uniform heat level and a successful quench would be straightforward without specialized equipment. If your annealing process is not consistent then you will end up with an irregular cylinder seal, which could send you back to Annealing 101.

I'd cop out and purchase a new aluminum spacer (gasket) and go worry about something else...
 
You must have found NOS alu gasket from old box or vendor wanting to be rid of it to a newbie/fool. There is very good reason they are rare as hens teeth nowadays they moosh into bores to rub on pistons. Won't happen very quickly but it will at some point then pistons may break. Alu washers are used on oil & brake banjos when annealed copper still too hard to moosh into cervices so leaking is not the issue with alu head gasket. Size of alu sheet needed for base or head tends to loose clamp loads too. Bad juju.
 
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illf8ed said:
I will be replacing the copper head gasket on my combat 750 with a thicker aluminum spacer shaped the same as a head gasket. Several reasons for this, but my question is regarding annealing aluminum in this application. Is it necessary and I see on line bring up the material to 775 degrees then slowly cool. I'm not inclined to go through this process. My plan is to use the spacer which by the way has been previously used and put silk around the cylinder bores, pushrod holes and one oil passage. Comments from anyone that has experience using these aluminum spacers to lower compression?
Click to expand...

Silk is not likely to withstand the head temps for long.
Aluminum is likely to creep or squish with time, temperature and clamp force .... more likely if annealed.

Use copper as oldmikew suggests.

Slick
 
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.
Click to expand...
The grade of aluminium the proposed spacer made from will have a bearing on it's suitability as a head gasket. What is it?
 
Do you want to anneal the aluminium spacer or age-harden it ? If you want it harder, you will need to soak it in a 100deg. centigrade oven. You already know how to soften it. If I was doing it, I'd harden the spacer and use a thin copper gasket each side of it. A stainless steel spacer might be better.
 
I wonder how much could be safely skimmed off the piston tops?

Dave
 
Hi Hobot,

I'm interested in your response. Do you have experience with aluminum head spacer compressing into the cylinder space? Aluminum has less chance of compressing than a copper head gasket and I have not had such experience with copper. You put some doubt into my plan to use this head spacer.
You are right about how I acquired it. Came with a stack of used copper head gaskets.
 
I'm interested in your response. Do you have experience with aluminum head spacer compressing into the cylinder space? Aluminum has less chance of compressing than a copper head gasket and I have not had such experience with copper. You put some doubt into my plan to use this head spacer. You are right about how I acquired it. Came with a stack of used copper head gaskets.

Yes very bad experience. Never heard of alu gaskets/'head spacer' till finding out what seized-locked rear destroying lots of internals after 3 wks mild use. Cylinder base gasket or spacer is factory approach to low CR. Later got Norton dealer releases to see it warned about. Expect more proneness to leaks with base spacer in between but a smear of goop. Herd wisdom is way to go on this. Realize any change in spacers/gaskets/machining alters pushrod length on rocker stem contact geometry for wear issues. May not ever be an issue w/o the comma oil hole - discontinued pistons stress risers so try it and see or more likely the next guy like me.
 
I got a 1/8 (.125) Copper head gasket made by a guy I found on eBay called "copper gaskets." He knocks them out on a CNC mill and then you have to anneal them yourself. His template/program isn't perfect so a bit of filing is necessary but the price is right. I did copper coat spray (over the whole gasket) and thread around the push-rod holes. I also use the CNW/ARP trimmed down stainless head bolts which stretch a bit when the head gets hot and then keep it pulled down when things cool off. No leaks so far and copper holds it's shape way better than those old aluminium head spacers.
 
If I remember right, The original push rods in the combat motor were about .020" too long.
A base gasket might help your rocker geometry.
 
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.
 
Due to the unavailability of dished pistons, I used a cylinder base spacer on my Atlas to keep the compression ratio down. I also put 'lash caps' on top of the valves to bring the valve geometry back into line, they add 0.04" to the valve stem length. Just as an aside, the cylinder base spacer for a commando will not work on an Atlas, the return oil hole is 1/3 obscured, Andover Norton make one for the early 750s. I've had no issues with this arrangement and I would have thought it much less likely for leaks of any sort at the base as opposed to the head. The lash caps came from RGM, part no 050465.
 
Gilesy said:
Due to the unavailability of dished pistons, I used a cylinder base spacer on my Atlas to keep the compression ratio down. I also put 'lash caps' on top of the valves to bring the valve geometry back into line, they add 0.04" to the valve stem length. Just as an aside, the cylinder base spacer for a commando will not work on an Atlas, the return oil hole is 1/3 obscured, Andover Norton make one for the early 750s. I've had no issues with this arrangement and I would have thought it much less likely for leaks of any sort at the base as opposed to the head. The lash caps came from RGM, part no 050465.
Click to expand...
Correct way to approach this job imo. Spacer plates made from an unknown grade of material that also require sealing on both sides in the hot, high pressure zone is shoddy work.
 
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