# what size tap for R2800 spark plug holes



## partsgone (Dec 12, 2021)

HI, I have had Pratt & Whitney R-2800 for many years and just today found a case of ww2 sparkplugs. They are not going in and i can tell if they are a different thread but need to chase the threads on the engine anyway. Does anyone know what size tap the spark plug threads use on the R2800.


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## MiTasol (Dec 12, 2021)

Most R-2800 engines used the Champion C34S and C35S series plugs which I believe are 18mm threads but all my references only cover the harness thread.


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## partsgone (Dec 12, 2021)

thanks, the plugs i bought are for Allison engines, but i also read they may be the same fit. I don't want to force anything and need clean spark holes anyway. I'll have a better look in the light.

scott


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## GrauGeist (Dec 12, 2021)

The Allison V-1710 used the Champion R33S, which has an 18mm thread.

They would work with the R-2800, though you need to be careful of the specified heat-range of that particular engine.


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## partsgone (Dec 12, 2021)

thanks, these plugs are all in boxes marked TYPE LS-4AD-1


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## GrauGeist (Dec 12, 2021)

Sounds like an AC Delco (general motors) number.

Might do a search with that part number and look for vintage application charts - they'll most likely be in .PDF format.


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## MiTasol (Dec 12, 2021)

Hi Partsgone - can you drop a photo of the boxes in please

Both AC and BG use LS part numbers but *most* LS series AC plugs were only LS with two digits and *most *LS series BG were LS with three digits. I have never seen an exception to that in LS series plugs but Murphy happens. The only BG plugs I can remember with two digits were their Allison plugs which were C34S.

I have never seen an AC or BG plug with a -1 not that that probably means much other than the dash is unusual. Maybe, but only maybe, it indicates a rebuilt plug (they did overhaul them) or oversize for worn threads but those are only a guess.

I have also never seen a BG plug with a mixed number like LS-4AD-1 which makes them interesting


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## GrauGeist (Dec 12, 2021)

Here's a PDF of a V-1710-29 service bulletin that states that the recommended spark plugs are:
LS321 (BG)
LS-4AD (Aero)
LS-3AD (Aero)

So the part number they posted is correct.


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## MiTasol (Dec 13, 2021)

Thanks - I had forgotten all about Aero - they were not around for long unlike the major players and I have never physically seen any of their products, just the odd reference in a document.

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## partsgone (Dec 13, 2021)

Yes they are Aero Spark Plug .co. NY NY.
can drop photo in later today. thanks.

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## GrauGeist (Dec 13, 2021)

MiTasol said:


> Thanks - I had forgotten all about Aero - they were not around for long unlike the major players and I have never physically seen any of their products, just the odd reference in a document.


You're welcome!

When I first saw the OP's part number, I thought "AC Delco" because of the LS 4×××, then I got thinking about AC's LS series which, of course, have just two digits, so I dug a bit and discovered I had forgotten about Aero, too.


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## partsgone (Dec 13, 2021)

Aero packaging

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## MiTasol (Dec 13, 2021)

Thank you

Looking inside the harness end what colour is the inside of the plug?


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## partsgone (Dec 13, 2021)

They are in the barn. i'll get back to you in the morning, if ok.
scott


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## GrauGeist (Dec 13, 2021)

Love the brass thread protectors.


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## partsgone (Dec 13, 2021)

top notch right.


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## partsgone (Dec 14, 2021)

Looks just like a brass primer in the center. The walls were coated with looks like super thin glass which was chipping out. It that stuff was needed then the plugs are shot.


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## MiTasol (Dec 14, 2021)

That stuff that is chipping out is mica (a rock) and yes they are shot. This makes them early 40's plugs as the ceramic (white centre) plugs replaced mica plugs at that time because mica is so fragile. I cannot remember what years.









Mica - Wikipedia







en.wikipedia.org

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## RichardSuhkoi (Feb 1, 2022)

MiTasol said:


> Hi Partsgone - can you drop a photo of the boxes in please
> 
> Both AC and BG use LS part numbers but *most* LS series AC plugs were only LS with two digits and *most *LS series BG were LS with three digits. I have never seen an exception to that in LS series plugs but Murphy happens. The only BG plugs I can remember with two digits were their Allison plugs which were C34S.
> 
> ...


I have part number BG Ls321A as a plug for Allison V12. 
I would be very careful chasing threads. If you use first a nylon bristle brush and thin oil you should get the dirt out. If you damage the threads you risk a spark plug exiting the engine when at high power. 
I don’t know if there is a rework using thread insert but the rule on boat engines etc is adding a threaded insert reduces heat transfer so you need a cooler plug. 
Champion make plugs for PW 2800 today.


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## RichardSuhkoi (Feb 1, 2022)

RichardSuhkoi said:


> I have part number BG Ls321A as a plug for Allison V12.
> I would be very careful chasing threads. If you use first a nylon bristle brush and thin oil you should get the dirt out. If you damage the threads you risk a spark plug exiting the engine when at high power.
> I don’t know if there is a rework using thread insert but the rule on boat engines etc is adding a threaded insert reduces heat transfer so you need a cooler plug.
> Champion make plugs for PW 2800 today.


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 ).


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## FLYBOYJ (Feb 1, 2022)

RichardSuhkoi said:


> 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.


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## FLYBOYJ (Feb 1, 2022)

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


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## RichardSuhkoi (Feb 1, 2022)

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

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## RichardSuhkoi (Feb 1, 2022)

FLYBOYJ said:


> 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



FLYBOYJ said:


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





FLYBOYJ said:


> 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.

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## FLYBOYJ (Feb 1, 2022)

RichardSuhkoi said:


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


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).


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## partsgone (Feb 1, 2022)

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


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