Perkins 1408 Heat exchanger bolts thread

[huldah]

Hi, during removal of heat exchanger, the bolt on the rear end broke .I have successfully drilled out the old bolt leaving a thin layer of metal in the thread inside. I need to identify the thread so I can clean out the old metal. The OD is approximately 7.9mm. It has approximately 21 threads per inch. I assume this to be an American thread. Presumably these taps are still available, if so, where do I obtain them?

 

[Motor_Sailor]

Hi, during removal of heat exchanger, the bolt on the rear end broke .I have successfully drilled out the old bolt leaving a thin layer of metal in the thread inside. I need to identify the thread so I can clean out the old metal. The OD is approximately 7.9mm. It has approximately 21 threads per inch. I assume this to be an American thread. Presumably these taps are still available, if so, where do I obtain them?

Well they will be imperial, and 7.9 is about 5/16 of an inch.

But UNC is 18 TPI and UNF is 24 TPI. Try counting again!

You'll probably need to Helicoil it if you drilled the bolt out. There's plenty available online

 

[huldah]

Well they will be imperial, and 7.9 is about 5/16 of an inch.

But UNC is 18 TPI and UNF is 24 TPI. Try counting again!

You'll probably need to Helicoil it if you drilled the bolt out. There's plenty available online

Thanks, I used a gig and there appears (as far as I can see) to be no damage, only a very thin coating of the old bolt preventing it's reuse, this has moved slightly inwards, giving a good start for a tap. It looks as though this must be UNF, as a more careful measurement shows the threaded area to be less then 1".