How smooth is too smooth for bearings and journals?

[Cronik]

Just in the process of rebilding a toyota DOHC 1.6L motor and i'm thinking about how i can make this engine run a tad smoother and quieter. One of the things i was looking at was improving the surface on the cam bearings and journals and tightening up the clearances.

Is there a point where the surface just becomes too smooth to allow for the formation of a proper oil film?

I know i can polish the cam bearing surfaces to a mirror finish if i need to, but would this be actually be a god thing? or am i better off just giving them one of two quick passes with 400 grit wet and dry after the align bore?

I have been reading a few articles on the web about micropolishing of cranks down to as low as 4RA. Is there any benefit to going this fine?

 

[ivymike]

Is there a point where the surface just becomes too smooth to allow for the formation of a proper oil film?

for a squeeze bearing, yes. For a journal bearing, probably not.

 

[Latexman]

Why don't you use electropolishing? The abrasives are at the molecular level. You cannot get smoother than that.

Good luck,
Latexman

 

[evelrod]

I agree that time spent polishing up crank or camshaft journals, so long as minimum material is removed, can be an uplifting and often benificial experience...you feel like you are "doing something productive". In reality, standard finishes on journals is sufficient in almost all applications. I usually make sure that any final polishing is done so that microscopic imperfections 'point' in the right direction, away from the direction of rotation (makes ME feel better, too). I DO NOT approve of the common technique of using a Scotch Brite pad to 'polish' the new bearing surface...I just measure each shell and install them 'as is'. You will surely get other opinions from equally experienced engine builders...I don't do it.

As to "tightening up the clearances"...I do DOHC engines on a regular basis and have done a Toyota 2TC and one 18RG DOHC using standard clearances specified by TRD at the time...no problems that I heard of. I hold my 1600 twincam Fords to one and a half thou on the rods and mains and a bit less on the cam journals, 83.5mm JE pistons go in at a bit less that three thou at the skirt ... I use 40 Redline racing oil.

Rod

 

[Tmoose]

I think you'll find most manufacturers suggest RMS 10 uin or better. The races,balls and rollers in antifriction bearings are at least that smooth, and have some regions of sliding friction. A new valve lifter or valve shim is pretty smooth and 100% hydrodynamic. Even when plateaus and other oil retention schemes are mentioned there a smooth flat area after break-in. Dulled 400 grit makes a lot smoother feeling finish than new 400. Directional polishing is important on cast and nitrided crankshafts.

A smooth journal with much more than 0.0001 inch taper or out-of-round is inferior to rougher one with better geometry. It is possible to polish a vew 0.0001 taper into a journal ny hand, and easy with a motorized crank polisher.

 

[Fabrico]

Messing with the bearings will do nothing to make the engine run smoother and quieter. About the only thing that might help is having internal and external rotating parts precision balanced, and careful checking of internal dimensions (blue printing). This could include some of the accessories. There may be some "better" motor and/or exhaust mounts.

The OEM's spend considerable effort in this area. It is doubtful you will make significant improvements.

 

[GregLocock]

I'd reinforce the message about directional finishing. It can be very important.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Cronik]

Thanks guys, i'll have a read and a think.

I was aware of the importance of not tapering the journals and will be keeping a very close eye on this. Hopefully the crank grinder returns it with a good finish and i wont have to spend much time improving the finish. I get the feeling by the end of this rebuild i'll be reciting clearances in my sleep..