offenhauser head 39-48
 

I know someone on this site can tell me which spark plug to use with these heads--looks like I need a thread length of 5/8" but have had not luck finding anything to fit. these heads may be the pre 2004???mfg.

Re: offenhauser head 39-48
 

I use Autolite 437.

Re: offenhauser head 39-48
 

won't work, threads are not long enough

Re: offenhauser head 39-48
 

If the heads are an older set they would take the standard flat head spark plugs Champion H-10 Autolite 216 NGK B4L all replacement plugs for the flathead V-8. Later castings by a few other brands use a 3/4 inch thread. Never had an issue with older offys using any and all the above plugs however I prefer the NGK they work the best from my experaince.

Re: offenhauser head 39-48
 

I purchased new Offy 425 heads for my 59A/B two years ago. The plugs from the stock cast heads were Champion H12, whose threads are 3/8" from the washer to the end of the threads. The H12's were too short in the Offy, leaving three threads of the hole exposed, and the tip back up in the spark plug hole. I tried a hotter range plug with 1/2" threads, but that put the tip pretty far out in the breeze. Settled on Champion L82C Copper Plus plugs which are a hotter range and have 7/16 threads. That length leaves about one thread in the hole exposed. The engine runs great on these plugs; no missing, no pre-ignition. But I might try a cooler range plug, as the color on these is a dusty gray instead of the toasty brown I like to see.

Re: offenhauser head 39-48
 

They work fine in mine. Previously used Champion H-10.

Re: offenhauser head 39-48
 

it is a pretty lousy design on the part of offy to have designed a head with 5/8" thread reach and NO plugs that are actually made for them. yes I know these will work but they are NOT correct. Too much is made now that has parts that are not CORRECT but can be made to work. I guess the Chinese will own us before it is over, if the don't already.

Re: offenhauser head 39-48
 

Please let us know if you find the correct ones.

Re: offenhauser head 39-48
 

What's not correct is the heads the plugs are correct. But if you feel its that important to use all the threads as cast then your not going to be happy. One solution is to spot face the plug hole from the spark plug side removing enough aluminum so you eliminate the threads not used just a thought but not really necessary.

Re: offenhauser head 39-48
 

I use a 1/2 counter bore to remove the unwanted threads. Takes about 5 minutes.

Re: offenhauser head 39-48
 

This is an age old problem with Offy heads. Use a chamfer tool(like used for preparing a hole for flat head screws) and chamfer the excess threads out of the combustion chamber.