Main bearing cap bolts, oil or no oil?

weebur said:

I think that most torque values are based on a dry fastener. Lubing a bolt/nut can cause over torquing due to less friction, but maybe an engine builder can jump in here. If a fastener is difficult to run down by hand I usually run a tap through the hole or a die over a stud/bolt.

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Alexia said:

Thanks to both. I am mainly asking since I see all the guides out there for rear main seal replacements that just say to put the bolts back in, but nothing about cleaning the oil off them.

Granted, this is a complete rebuild with a hot tanked engine block. So everything is sparkling clean!

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5-90 said:

"clean, dry" threads UOS are the norm. If the "book torque" value is specific to a given lubricant, it will so state (examples are the #11 cylinder head screw and the crankshaft nose screw.)

However, I would make sure to do the following:

- "Chase" the threads in the housing. You don't need a tap to do this, and you're better off not. The screws should be threaded 1/2"-13 - go get a couple of 1/2"-13 SAE8 capscrews, and use a thin cut-off wheel (1/8" will be fine) and cut 2-3 longwise grooves across the threads. Run the screw down to the bottom and back out again - clean out the grooves you cut, and repeat. Clean all holes. (A tap can let you screw up and cut new threads, if you're not careful. That is far less likely to happen if you use screws. You can also purchase purpose-made thread chasers, but they can be made just as easily and far more readily.)
- Use LocTite #242 on those screws! The very last thing you want them doing is working loose! (LocTite is formulated to not lubricate, no change from "clean, dry" spec is desired or required.)
- Given a choice, I like to clean the heads & caps before they go in, install & torque, check torque, then paint a bright mark on the head and cap - this will many any loosening of the screw immediately visible. You can get purpose-made paint for this (it will visibly crack and flake if disturbed, it may be called something like "Torque-Seal" or something. Check MRO houses, race supply houses, and G/A supply houses like Wicks Aircraft or Aircraft Spruce & Supply for this stuff.) It saves you having to drill for safety wire, and provides an immediate and obvious indicator of loosening on inspection. I do the same thing with bottom engine engine fasteners - connecting rod nuts and main bearing cap screws - so if I'm pulling down the oil sump, I can tell if anything else is awry (do I need it often? No. Does it save me time? Yes. The extra five minutes spent doing this on each engine assembly - probably 10-12 hours all up - has saved me umpteen hours on inspection - probably at least a solid week, or 168 hours - over the years. Yeah, I'll keep doing it!)

Fasteners should be able to be run down to contact by fingers alone - if you can't do it with 2-3 fingers, pull the fastener and clean out the hole. You shouldn't need a torque wrench to start the screw, just to finish it.

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weebur said:

Thanks for your input, I was hoping you would jump in.

I guess I should have included some things about using taps. Back about 1,000 years ago when I worked in the Tool and Die field for a few years I learned about using taps in steel. Breaking a tap in a $10,000 die can have you seeking new employment pretty quick.

I could go on and on about using taps, but this is probably not the place for that. I will say that I like your idea about using grade-8 screws instead of taps, especially for the inexperienced. Taps are damned near as brittle as glass and will break unless you use the proper technique. Breaking one below the surface can turn a good day into a very bad one.

And, thanks for the info on using LocTite, I wondered if it acted as a lubricant.

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5-90 said:

Yeah, breaking taps is a pain.

A few years back, one of my boys broke off a tap in the nose of the crankshaft on his car. He was getting ready to pull the transmission to drop the crank and have it machined out.

I went out with a propane-oxy torch, had him hold a water hose, and burned it out. Took about a half-hour, and didn't even damage the front main seal.

Ya just gotta be careful...

Shall I assume that you'd broken off that tap before EDM came along? I'm told that makes the job so much easier (I used the oxy torch for a few reasons - I'm familiar with flame, it had a very fine flame, and I could use it without having to pull anything to bits...)

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5-90 said:

If you check the introduction of your FSM (or whatever service manual you're using,) you'll probably see something to the effect of "All torque specifications given in this manual are for 'clean, dry' threads unless otherwise specified." (that's what "UOS" means, I forgot to mention. Similar abbreviations I use are "NOS" - Not Otherwise Specified, or "UNODIR" - UNless Otherwise DIRected. They mean essentially the same thing.)

So, you'd at least take a rag to the screw and blow out the hole with some carburettor cleaner, just to get the oil out. See my notes elsewhere on lubricants affecting torque/tension relationships.

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