How far can I drill

[Nickodell]

I stripped the thread on the socket into which the top bolt for the alternator adjustment arm goes, and I'll have to drill it out oversize and re-tap from 1/4" to 5/16". OK so far, except it's an awkward sod of a job requiring the removal of the radiator and top belt pulley.

However, on removing the old bolt I notice that only 1/4" of the 1 3/4" bolt is actually threaded into the cast iron of the block - the rest spins free in the aluminum timing chain housing it has to pass through. A quarter of an inch is a very small amound of "meat" for the torque required, which no doubt is why it stripped so easily.

Question: Does anyone know how much further I can drill into the block to deepen this hole before penetrating water jackets etc? This is especially important as it is a blind hole, making it difficult to tap threads even to the end of the already inadequate 1/4."

 

[sunbeam74]

How deep can you drill

Nick,

Are you sure there isn't a bolt broken off in the hole? On my race car after the 2nd year of running Mt. Washington, a very rough road, I had that bolt snap on the alternator/waterpump due to vibration. Fractured the bolt about 1/16"-1/8" down in the hole. Just about where the shoulder ended and the threads started.

Technically, assuming we are talking about the same hole, the hole is .8" deep. I am betting there is the remainder of a bolt in that hole if you only measure .25" deep.

After I had the bolt snap I made very sure the lower mount bolts were tight with nyloc nuts. Otherwise it transfers all the stress and vibration to that upper bolt. I also checked the lower bracket for cracks - VERY common problem.

Steve

 

[Ken Ellis]

I'll have to second Steve's remarks. It's more than 1/4". I had a broken off bolt (same place) that I put off attending to until I had the radiator out for replacement. Stocked up on ez-outs, etc., expecting an exciting session of broken bolt removal. Went to drill the ez-out hole, and the entire bolt had rusted to dust. Took about a second to drill out the whole thing. Still had to enlarge and tap the hole in the block because the outer threads were messed up, but it was pretty easy. Ended up using a right-angle drill to get the tap started, but that was the only "trick" of the job... that and getting the hood back on where it used to be...

Ken

 

[Nickodell]

Hi, Steve and Ken. You guys were right! I took out the radiator and removed the fan pulley (surprising how fast you can do this after a few times) and proceeded to drill out the hole. I decided to go up to 3/8" so that I'd remove all traces of the original threads in the wall of the hole. After going in easily for 1/4" (cast iron) I encountered very firm resistance, and then switched to a much smaller drill and tried my Ez-out (screw extractor). No dice; I tried to turn it until I was afraid that I would break it (one of the absolute worstengineering nightmares you can have, as Ezy-outs are so hard you will never drill them out!

So back to the 3/8" drill. The shavings coming out were the typical curly ones from steel, rather than the small "filings" you get when drilling cast iron. After grumbling and grunting for 10 minutes (on and off) removing what was obviously some 1/2" of broken steel bolt, the drill suddenly went through and proceeded with no resistance for about another 3/4 inch. "Aagh!" I thought. "I've broken into a water passage!" I quickly replaced the bottom hose with the end pointing up, and filled the block with water up past the hole I'd drilled, holding my breath. No water leak!

The next trick was working out how to tap the hole when there was no room for a tap wrench to turn. By juggling with small metric sockets and drivers I managed to come up with a makeshift rig and the job was done. I decided to insert a home-made stud instead of using a bolt again, so that I can remove and replace the alternator adjustment bracket without continually unscrewing and rescrewing a bolt and risking stripping threads again.