what size tap for R2800 spark plug holes

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partsgone

Airman
21
4
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.
 
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
 
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
 
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.
 

Attachments

  • AllisonSpec132-B,V-1710-F2L(V-1710-29).pdf
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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.
Click to expand...
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.
 
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.
 
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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