gm 840 rectangular port heads spark plug engagement

[dynoflo]

finishing up rebuild of a 468 bbc I was putting plugs in and noticed only 3 threads of engagement on head. Engine came in with no plugs so i looked up what it called for and found AC delco r44xls. Head looks like about 6 or 7 threads. These heads are over 50 years old but are in good shape. Anybody run across this before. Thanks in advance.

[Baprace]

I am going to guess , you need peanut plugs with taper seat, r44xls are big diameter plugs 3/4 reach with gaskets , the thread is the same on both types. The body on r44xls is a larger diameter , 13/16 socket. There is a small diameter plug that uses a gasket and uses 5/8 wrench BUT I don't have a part number.

[Old School]

I take it the heads are already on the engine. If not look and see if the end of the plug is flush with the combustion chamber. All 840's I have seen use the 13/16 plug, 3/4 reach with a gasket. There is a 5/8 plug, 3/4 reach with a gasket that can be used to make getting the plug in and out easier.

I don't think any 840's were made for the tapered plug. The 840 was primarily a 1968-69 head although I have heard of them being on a very late date 1967 engine. Best of my memory the tapered plugs were first used in 1971.

Bend a L shape on a small wire. Put the wire in the plug hole and hook the L on the inside of the chamber. Mark the wire where the flat on the head is for the gasket. Then measure and see if your plug is the right length and going the correct amount into the chamber.

[BP758]

My 840 takes the Autolite3910 .750 reach with a gasket.