Engine block fasteners 1

[tcp2000]

My problem is, while trying to remove the cylinder heads from a 1993 Isusu Rodeo I managed the break one of the 10mm bolts that attach the heads to the block. The bolt broke off about 2 inches inside the block. I tried a bolt extractor and was unsuccessful at removing the broken bolt. Both the heads and the block are composed of aluminum and I believe the factory bolts are also.

My idea is, if I am unable remove the broken bolt I would like to tap (currently untapped) the first 2 inches of the bolt hole to accept the next larger (and much shorter) fastener and leave the broken bolt in the block. The head has an opening large enough to accept a larger fastener (without drilling it) than was being used.

My question is, why did the engineers use bolts that protrude 3.5 inches into the block and only have 1.5 inches of thread at the very end? What is the advantage of using long fasteners with threads at the end verses fasteners that are shorter but threaded all along the length? Also can I “get away” with tapping the existing bolt hole (it is in the middle of the block not on the end of it) to accept a shorter fastener without having the head leak or warp at a later time? Any help in this will be greatly appreciated.

The engine is a 1993 3.2 liter V-6, dual overhead cams, 4 valves per cylinder, aluminium block and heads. The fasteners in question have a torque of 47 ft lbs.

 

[GregLocock]

The cylinder head bolts will be steel, not aluminium.

Try to find someone local who can do electro discharge machining - which is very easy for a workshop to do, to remove the old bolt. Have you tried heating the block with a blow lamp and freezing the bolt with CO2 or nitrogen?

Failing that, your proposed solution is pretty nasty, but I've done worse. You could even helicoil the new hole.

"My question is, why did the engineers use bolts that protrude 3.5 inches into the block and only have 1.5 inches of thread at the very end? What is the advantage of using long fasteners with threads at the end verses fasteners that are shorter but threaded all along the length? "

Ok, in the case where you have similar materials for bolt and block only about the first three full threads actually do anything. In the case of a steel bolt in an aluminium block rather more of the threads do contribute, but (at a guess) it would still only be 10 threads or so. If they'd used fully threaded bolts 3.5 inches long you would never be able to undo them, any corrosion at all would weld them into the block. It's also difficult to tap long threads.

Why did they use long bolts, not short ones? because long ones allow the clamping force to be better distributed into the structure, and there may be some issue with thermal expansion. Long threaded joints are generally more robust (as regards torque relaxation) than short ones.

Cheers

Greg Locock