Loctite Compound to Prevent Fretting Between Parallel Surfaces... Effect Bearing Clearance?

[SynViks]

The gist is using a loctite compound between the main bearing caps and block on an engine to help prevent fretting/cap walk. The concern I have is will using a compound throw off bearing clearances/create an out of round situation orthogonal to the crankshaft?

I had a short conversation with Henkel who suggested 640 for this application. That's more-so used for retaining bearings and keeping cylindrical press/slip fits from from rotating within their bore. Based on the TDS it looks like it would hold up to the oil and 200F temperatures. It's very low viscosity.

I know that cummins uses 518 in their ISX platform:

This seems to be a thicker compound used for flange gaskets, and the TDS makes mention of lap joint shear strength while the 640 does not. They warn about using "other sealants" as they could become brittle and cease to function. I don't know if 640 was considered within the context of that statement though as it's not a sealant.

So the big question is two-fold: which compound would better suit the heat, oil, and the high number of cyclical forces that the bearing cap will endure, and will using one of these compounds add "thickness" between the mating surfaces, causing an out of round condition? For context, the diameter tolerance for the bearing when the caps are torqued down is two thousandths, and there are four fasteners per cap delivering a clamping force of 22,700 pounds each.

 

[markz528]

I just don't see it. My experience is not with a Loctite product, but same concept. Used in electric motors with large ball bearings. Did not work. Did it help - don't know, but still had some fretted bearings.

 

[ratbldr427]

Do you have a problem with fretting at the cap joint? That is pretty unusual. Any thing you put between the surfaces has a thickness , how much before it is a problem? Will it impede heat transfer? If you are a real stickler for precision then align hone with the compound in place. Remove and replace the Loctite during the build. These days it is possible to out put many times the oe horse power so don't know if it would help any thing or not.
I pretty much go by what the factory recommends simply because they have done the research and testing. I find it strange that an oe would design a bearing cap that required it unless the problem cropped up in the field and this is a patch? They can at design time make the lower end as robust as necessary unless they screwed up on this one.

Many years ago(60 's) some builders tried using Loctite between the bearings and cap. It affected the clearance some . But was a cure looking for a problem.

If the surfaces are fretting then something is not holding. The registers in the block should be sufficient to stop any movement along with the bolt torque.

 

[PackardV8]

First, no sticky liquid would stop fretting between block and main caps. Find the cause and fix it. We'd first verify the caps and block register is still tight. If correct, nip the caps and line hone the bores. Then a professional balance of all rotating/reciprocation parts. Clean the bolt/stud threads with the proper thread restoring taps and dies. Use a new OEM front balance damper. Assemble using a recently calibrated torque wrench. Dyno verify the tune/govenor settings.

Second, I've spoken with Clevite engineers and they state categorically never use loctite between bearings and housing bores.

jack vines

 

[SynViks]

To be clear this wouldn't be the only tactic of preventing cap walk. It'd be in concert with plate girdle, ARP studs, balance job with ATI damper. It's more like, if it can't hurt and might be able to contribute that extra 10 or 5 percent, why not. This build should be outputting around double the torque of OEM specs so every little bit helps. Also, no plans to put compound between bearings and caps.

I'd have to do some testing with similarly finished machine surfaces and similar clamping forces to see if the compound produces a substantive change in thickness. Idea is the Loctite would mostly settle in the machine mark valleys and help bond those "dips" to the opposite piece, adding resistance to the microscopic parallel shifts the caps might be temped to make at high cylinder pressures.

Don't really have time to do that testing, so my auto machinist and I have agreed to nix the idea. Was just curious if it'd been tried by others. I appreciate the thoughts others have contributed.

 

[PackardV8]

You've thought through the obvious improvements. Two others to consider are alignment sleeves around the studs spigoted into the caps or alignment dowels mating into the solid area of the caps. Either will make a major reduction of cap fretting.

jack vines