Offenhouser Head Repair

P

paleot

Senior Member
I bought a set of offy 375 heads at swap meet, one head has two stripped plug holes . The two have been tapped to what looks like 18 mm and stripped again. what is the best way to repair these? Can they be welded drilled and tapped back to 14 mm?
Tony
 
At worst you could have them welded, drilled, and tapped.

Heli-Coil sells a plug hole repair kit. The kit uses inserts instead of coils. It comes with a combination reamer/tap that threads into the 14mm hole to start. The one problem is that your 14mm hole doesn't exist anymore. It might be worth a shot, though, because if you can start the reamer part squarely in the hole, the tap may engage properly.
 
It CAN be fixed without welding.
First, determine what tap size will go in the existing hole and leave a set of CLEAN threads.
A taper pipe thread is best but standard straight will work ok.
Next make an aluminum plug on the lathe with that thread on the outside.
Leave the plug long enough to grab it with something to drive it in the hole with Loctite. Cut the plug off and spot face the area flat. Drill and tap a new 14mm hole.

Welding on those old heads will leave the area soft and susceptible to problems down the road.
 
paleot said:
I have the master helicoil repair set for 14 mm I tried the tap and it is too small for the hole.
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The reason your master tap falls through the hole is because that hole has been stripped larger than 18mm. There is a company that makes oversize inserts. Take a look at this site and then give them a call.
http://www.timesert.com/html/bigsert_sparkplug.html

Pete's idea is excellent . If you are careful you could make the threaded plug the correct length but mark both ends with a small center drill and then drill a 1/8 inch dia. "through hole" so that you will be exactly "on center" when you drill and tap your hole for the spark plug threads. I fact, you may be able to make the 14mm. spark plug internal threads on your plug while it is still centered in the lathe chuck. Then screw the newly made plug onto the end of an old spark plug and press it into the cylinder head as Pete recommended. That way. you know for sure that the new spark plug threads are "right on". Don't forget the red Loktite. Use steel to make your plug.
 
I believe Pete recommended EXTERNAL threads on the manufactured insert...needs to be "threaded" into the NEW hole...not "pressed". You knew that, though. DD
 
Years ago I did allot of aluminum cylinder head repair and the old heads were cast with poor quality material and welding was difficult at best. Of all the different methods, Pete has the answer and is more cost effective.
 
You might want to make sure the 375 heads are what you want to use before you go any further..check your clearances on valves pistons etc
 
V8COOPMAN said:
I believe Pete recommended EXTERNAL threads on the manufactured insert...needs to be "threaded" into the NEW hole...not "pressed". You knew that, though. DD
Click to expand...
Opps! I incorrectly read Pete's suggestion. Thanks for letting me know. Getting old is no fun.
(Plus, my headlight relay is still working fine.)
 
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