what size tap for R2800 spark plug holes

[FLYBOYJ]

Question:
Does anyone know if the TO for Allison required spark plug gaskets to be replaced at each inspection? The idea being these get crushed and ever time you torque the igniter, then run the engine, they cold flow so if you re-use you get excess plug penetration , tip temperature, then detonation.
Reason I ask is I have a jar of gaskets but if nobody swaps these I will assume they are useless (each spark plug comes with a new gasket I believe ).

Just out of best practices, I've never re-used them.

 

[FLYBOYJ]

I now remember, you can anneal them if you want to re-use them

 

[RichardSuhkoi]

Thank you.
I'll clean and inspect these and try to get them to users.

 

[RichardSuhkoi]

Just out of best practices, I've never re-used them.

With this info I'll clean, measure, and then find a guy with an Allison

I now remember, you can anneal them if you want to re-use them

I now remember, you can anneal them if you want to re-use them

This is an inappropriate procedure. I'm an ignition person at PWC and investigated this practice maybe 25 years ago (on turbine engines but this applies to piston). The copper gasket does not heat treat, it work hardens. So when you take a new soft gasket and crush it, it stiffens as it's crushed and deforms plastically. The spark plug stops advancing into the engine once the gasket has work hardened to the point it has the strength to hold back the crush. Once the engine runs, it will re-soften. Not fully anneal, but greatly soften. This happens at just 275 degF or so. If you re-install it will flow plastically again and become thinner. Fully annealing is slightly worse. This is easily proved by measuring thickness new, then running the gasket, remove, anneal, install, remove, measure.
The problem here is spark plug tip temperature increases greatly with penetration. Risk is preignition at high power.

 

[FLYBOYJ]

With this info I'll clean, measure, and then find a guy with an Allison



This is an inappropriate procedure. I'm an ignition person at PWC and investigated this practice maybe 25 years ago (on turbine engines but this applies to piston). The copper gasket does not heat treat, it work hardens. So when you take a new soft gasket and crush it, it stiffens as it's crushed and deforms plastically. The spark plug stops advancing into the engine once the gasket has work hardened to the point it has the strength to hold back the crush. Once the engine runs, it will re-soften. Not fully anneal, but greatly soften. This happens at just 275 degF or so. If you re-install it will flow plastically again and become thinner. Fully annealing is slightly worse. This is easily proved by measuring thickness new, then running the gasket, remove, anneal, install, remove, measure.
The problem here is spark plug tip temperature increases greatly with penetration. Risk is preignition at high power.

Good to know! I remember guys doing this when I was turning wrenches in SoCal. To me it was just too much of a hassle and every time I changed spark plugs I always bought new gaskets, especially for my airplane (when I owned one).

 

[partsgone]

I have a 100 or so plugs. looking for a mag and carb for a r2800-43. thanks for the info