MG Car Repair/MGB Tapping the Block
QUESTION: I recently replaced my 74 B cylinder head with a new cast version, but as I torqued one of the studs - it broke off in the head. Next, another stud pulled out what looks like helicoil but might be the original thread material. So all studs are for replacement, and all holes for re-tapping.
I am going to re-drill the head using 25\64" bit and then tapping and inserting a 3\8" helicoil to match the 3\8"\16 thread stud.
Can I drill out the existing holes in place? any tips on ensuring drill is straight and true?
ANSWER: Hi Paul.
I hope I have mis-understood you.
You are not going to drill out the holes in the new head, are you?
You must remove the head and deal with the block separately.
Before you start, stick some gaffer tape over the bores so no swarf can drop down into the bores.
If a hole is completely stripped, then drilling the hole may not be necessary.
I would not recommend using a Helicoil on a good hole. I would only restore a bad one.
The factory studs were 3/8 so I would only use a 3/8 drill, not 25/64.
Note that the bottom end of the stud is coarse threaded UNC but the top end is fine threaded UNF.
Buy some long inserts that match the new studs. They are available in different lengths.
Check your thread insert tap against a new stud to make sure they are the same thread.
Keep the tap absolutely vertical, turning with a light pressure only.
Do not tap the hole to the bottom. Only tap far enough for the inserts to be fully inserted,
allowing for the fact that the end of the tap is tapered.
If you have got it right, the new insert should go fully into the hole and stop after another 1/2 a turn.
Finally, "follow the manufacturer's instructions" is good advice.
Hope this helps.
---------- FOLLOW-UP ----------
QUESTION: thanks Barrie
yes I am drilling the block ( not the new head ) and you can see from the pic ( attached ) that the thread came out with the stud when pulled from the block, so I must assume the hole is partly stripped and not re-usable unless tapped larger ( but studs are 3\8" diameter so I need to add the coil insert right? ( repair kit picture attached )
Your photograph shows a piece of cast iron block that has stripped out of the block along with the stud. The material is not a stainless steel insert.
What you now have is a 3/8 diameter hole. If you drill it out any bigger you will remove some of the material that the insert needs to do its job.
Please do not clean the hole with anything bigger than a 3/8 drill. That will only weaken it, which you do not want.
I am pleased with your choice of insert. 3/8 inserts with a length of 1.5 times the diameter means they are 9/16 in length.
When using the tap, remember to lubricate it with a thin oil. Once it really bites, I prefer to turn it 1.5 times and then back it off a bit before cutting the next 1.5 turns of the new thread rather than just turning it. Remember to clear all the swarf from the hole afterwards using your 3/8 drill held in your fingers, not in an electric drill.
Hope this helps