thread repair options... without heli coils..

[wayne petty]

Heli coils are a great thing... There are hundreds of thousands holding each jumbo jet together...  

but what if they are just not right for the application...

like a stripped hole in the head for the exhaust or the intake mounting..

a stripped starter mounts.. in both the block and in the bell housings..


there are several options in solid thread inserts...    regular versions and self tapping...

the regular versions are great but sometime more expensive to install..
on a 3/8-16.. the outer thread pitch is like 9/16-12  how many of you have a regular 9-16 12 tap... and for doing blind holes... a 9/16-12 bottoming tap...   plus the oversized drill bit...

then there are the ones most engine rebuilders use... these are self tapping thread inserts...    you drill the hole to the proper size.. (don't loose the box or tag that came with them so you know what size drill to use)

take a long bolt of the proper thread size.. put a nut on it... thread on the insert till the threads are even with the end.. (not the slot)   tighten the nut against the insert... i usually hold the insert with a pair of channel locks or with am adjustable wrench by gripping it at the slot.

i then line up the thread insert and while keeping the alignment with the long bolt sticking out... put a wrench on the jam nut and turn the thread insert into the hole... it will cut threads as it goes in..   once in.. with the top of the insert flush or slightly below the surface.. loosen the nut and back the bolt out...  sometimes you have to hold the bolt head when you loosen the nut..

there is one more thing most people don't know about...

allen nuts...

the are great for some thread repair applications.. like stripped ford bell housing threads where the starter bolts to the aluminum bell housing... there is a lot of torque put on those threads when the starter is cranking...

i drill the hole from the back side..  but not all the way through... so the allen nut will be pulled into the hole and stopped from pulling all the way through by the undrilled section...


i have found over the years... the most popular sizes are 5/16-18, 3/8-16,  8mm-1.25, 10mm-1.5 ..

hope this helps.....  and there are still a huge number of places for heli coils...   just thought i would let everybody know about the options...