How Tight Should Thru Bolts Be?

The interference fit...not torque.

I have a leaking thru stud and decided to try SI 1290. It is very explicit that the interference fit between the stud and for should be .000 to .0035, any less and there's no seal and any more you can crack the webbing driving in the stud.

With the P02 stud I bought I have .003 interference (bore is .5008 and stud is .5038)...this seems like a lot for a 1/2" bolt, but within the .0035 allowance.

It went in, but it took a lot more effort than I was expecting. It could be I wasn't driving it as hard as I thought because I'm hammering around the other cylinder and being careful not to hit anything else.

All the research I've found online says that studs in new cases, which I essentially have with an oversized stud, can take some elbow grease. I just want to make sure I not jacking something up.

Seeking input from the overhaul gurus...

MOst of my experience is with steel, not aluminum. However, .003 seems tight for an interference fit. I am used to .001 to .0015 on steel. I am sure the aluminum stretches more, but this is outside my area of expertise. I am thinking that you may want to over torque the nuts a bit to be sure it is fully seated and then back off and re-torque to spec. I would be a bit concerned at being at the edge of the spec, though I am sure the engineers were comfortable with anything in the range. Just be sure to take many radial measurements to ensure that it truly is only .003 over.

If you have a mic, you can chuck the stud up in your drill press and knock down a thou with sandpaper. Need to go slow and ensue that you are removing material evenly across the stud. You only need to address the center area of the stud where it is an interference fit. Need many measurements, as often the stud is not truly round and will give different readings at different radial points.

On the lathe, it is typical for me to turn to within a thou or two on precision parts and get to final size via a file, as I don't have an industrially rigid lathe. More precise that way and often a better finish.

In the end, if it went in and your sure the torque pulled the case together, I am sure it is fine.

I would not be too concerned about it. Many aluminum properties are different than steel by a factor of 3, as is the .001 vs .003 interference noted. With aluminum youngs modulus being 1/3 that of steel, it takes 3x strain to get the same grip forces.

It went in...just harder than I thought it would. Could be all in my head though...I was striking with my right hand (I'm left handed), I was more concerned about control because of proximity to the #4 cylinder, and I was using a dead-blow hammer which spreads the force out. Maybe I wasn't hitting it as hard I thought.

I guess I'm over thinking it especially since I mic'ed the bore a dozen times to make sure it was within SI 1290 limits.

Interesting thing was the limit from the SI is...

Always use a thru-stud that will provide a .000 (line to line) to .0035 tight fit.

Click to expand...

...according to my calculations, at normal case operating temps, you could loose the interference fit with anything less than about .001 because the AL case expands larger than the steel thru-bolt...Even with .001 you'll loose the interface around 450 degrees (all depends on alloy types, and I doubt the case gets over 250). So...if the original design was a normal .001 over and the SI allows for a P02 (+.002) without reaming, it makes sense I'm currently at +3 thou and the limit allows up to .0035

I ran it yesterday and no leaks. Flying today. Clear and 55 so the airport should be lively this weekend.

Couple things I learned in this process:

• During next overhaul, dowels and o-rings for sure

• 
  The air flow is not very predicable so an oil leak in one location can build up in another. After ground running, it was obvious where the oil was coming from. After flying, it goes everywhere, and was actually building up near the alternator mounting pad, so it made it look like it came from the crank seal.

• 
  The combination of UV light a Phillis X/C made searching for the leak very easy. The newer the oil the more it stands out.

• 
  When you build a pre-oiler...make sure all the fittings are tight. A barbed fitting doesn't hold 60 psi. I ran out of hose clamps and though it would be ok. Makes a huge mess when it lets loose.