Subject: Re: Actually by jsup on 2008/1/12 13:41:19
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Food for thought -- the difference in thread pitch (threads per inch) is the critical item that determines what strength is left in the holes. The holes are definitely weaker by being re-threaded, but the amount the holes are weakened is a function of the difference in thread pitch between the two bolts and less to do with the 0.02 inch difference in diameter.
Think of it this way -- when the holes were originally tapped for 8 mm bolts, a certain amount of metal was removed.
Now the holes are re-tapped for 5/16". More metal is removed, now from different parts of the hole wall. Not a good situation.
You now put your 5/16" bolt into the hole and torque it. The threads are not supported completely -- if the holes have been weakened enough, the hole wall will fail before you reach the torque specification. That's probably the best thing to happen, as you will now have to bring the unit to a machine shop and have them repair the holes correctly. What is likely to happen is that the hole walls will support the torque spec, but will fail when the engine is under full load.
Why? There are gaps where the original 8 mm bolt tap had already removed metal. The RTV, or threadlock or any other additive is not going to increase the ultimate strength of the bolt in the hole. All they do is to resist the bolt from turning. However, when the load is placed on the bolt, it is straight out of the hole, the additives have no affect.
This is harder to explain than I thought, and I'd be happy to try again, but the bottom line is that I wouldn't trust your "fix".
I don't disagree with your assessment. My repair is just that, a repair. If it goes badly, I'll take it off and Helicoil the hole. THe threads were so trashed I didn't have much of a choice.
I'll admit, it's a shot. Not too much material came out of the hole, I know what you are saying about thread pitch. Essentially, if there isn't enough meat on the threads, the bolt can pull right out.
At the point I was at, there wasn't much I can do without complete dis assembly. So I took a shot. If it has to come back off, it's not that big a deal, it's external...
We'll see what happens.
why not drill them out and grow the hole to 9mm or 10mm? Just a thought. I don't know the area and have no idea if there is enough meat left.
Because it's on the car. I don't wan tall those filings flying around that could possibly go into the engine.
Tapping the holes was easy as the filings stayed in the oil mix that was on the tap. Drilling would be impossible as I couldn't get the drill perfectly straight and stuff would be flying. YES, I COULD drill nice and slow, but I can't get a drill in there, and a right angle drill isn't easy to control and I can make things worse.
It seems OK, it' not holding a whole lot, I think it's going to work fine.
I am looking for the spec on these bolts buy I'm expecting no more than 40 FT LBS. All they do is keep the gasket tight between the manifold and the runners.
