Damaged center bolt hole on cylinder barrel

Daytonachuff · 2026-07-02T21:34:50-0600

I removed the head off a 72 Commando. Whoever assembled it last did not install the center bolt on the head . The threads on the cylinder barrel look ok but distorted a bit from corrosion/heat. Was considering a Tap to try to clean it up except I’m trying to figure out the size to use. Has anyone done this ?

Torontonian · 2026-07-02T22:17:47-0600

The center bolt is the most important one . Also the most difficult one to access , a 6 " extender to a whitworth socket .

dobba99 · 2026-07-03T00:02:14-0600

Daytonachuff said:
I removed the head off a 72 Commando. Whoever assembled it last did not install the center bolt on the head . The threads on the cylinder barrel look ok but distorted a bit from corrosion/heat. Was considering a Tap to try to clean it up except I’m trying to figure out the size to use. Has anyone done this ?

3/8" x 26TPI

ludwig · 2026-07-03T02:34:26-0600

About that bolt: you would do well to replace it with the same as the outer 4 bolts, and an alu spacer to compensate for the difference in length. ( = 5 identical head bolts)

Carl H · 2026-07-03T04:08:29-0600

dobba99 said:
3/8" x 26TPI

Daytonachuff said:
I removed the head off a 72 Commando. Whoever assembled it last did not install the center bolt on the head . The threads on the cylinder barrel look ok but distorted a bit from corrosion/heat. Was considering a Tap to try to clean it up except I’m trying to figure out the size to use. Has anyone done this ?

Try to wire brush the hole and use brake cleaner and try one of the outer 4 bolts in the cleaned out hole. Most taps you buy will cut slightly over sized threads , and make the hole sloppy. A thread chaser or a old bolt made into a thread chaser is a better thing to use. You need to buy a few new bolts anyways, so you will have a used one to try and you can use it to chase out the other holes too!

concours · 2026-07-03T04:13:14-0600

Carl H said:
Try to wire brush the hole and use brake cleaner and try one of the outer 4 bolts in the cleaned out hole. Most taps you buy will cut slightly over sized threads , and make the hole sloppy. A thread chaser or a old bolt made into a thread chaser is a better thing to use. You need to buy a few new bolts anyways, so you will have a used one to try and you can use it to chase out the other holes too!

This man speaks the truth. I do this kind of thing every day in the industrial world, that whole pictured only needs to be cleaned or chased… Using a old bolt and sawing grooves on it should work for that very minor cleanup job.

kommando · 2026-07-03T04:56:11-0600

Sawing grooves, well on a slightly bigger thread.

Damaged center bolt hole on cylinder barrel

Damaged center bolt hole on cylinder barrel

Daytonachuff

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Torontonian

dobba99

ludwig

Carl H

concours

kommando

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