Second Attempt Thread Repair

[Citroënbender]

This motor, it’s in-chassis and clearance above is not great, particularly towards the back.

The four small holes down the centreline are our problem. They’re supposed to be about 65-70mm deep with an M6 thread, that’s torqued to 7Nm+90°+90°.

Three of the four holes are compromised. Broken off M6 bolts jammed in the bottom, and have been Helicoiled to M8 above this. Two of these have stripped their Helicoils.

There’s prevailing opinion that the long M6 bolts can be replaced by shorter M8 bolts at 25Nm (not angular torqued). So the M8 in itself isn’t a problem.

Common sense suggests unifying the fasteners, so that’s one drill/tap, probably three extractions and insert type repairs.

What I’m thinking is, make a drill jig. Starting with a length of 20mm 6061 flat bar, cut to fit just between the cam bearing blocks, and a couple of turned bosses screwed to it’s underside, that locate snugly in the larger holes of the centreline.

Then use drill bushes, to locate the small hole centres accurately. A small bore bush (3/16”) to extract the broken ends, and a larger bush for whatever insert is recommended. Both would have to be used with a right angle drill.

I guess the questions are: Is there a standardisation of drill bushing OD and length within certain size ranges, and is it too ambitious to try this in-chassis with depths approaching 70mm on the smallest drill, noting the right angle head?