Main bearing clearances
Main bearing clearances
(OP)
Hi,
Does anyone know how oil thinkness affect main bearing clearances. It has been discussed before here thread71-134618, but not is such detail
For example how much clearance must one run if one uses 5W, 10W,.. 30,40, 50 grade oils?
Thanks
Cheers
Ziga
Does anyone know how oil thinkness affect main bearing clearances. It has been discussed before here thread71-134618, but not is such detail
For example how much clearance must one run if one uses 5W, 10W,.. 30,40, 50 grade oils?
Thanks
Cheers
Ziga





RE: Main bearing clearances
I've probably forgotten something.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Main bearing clearances
Balance of oil flow between journals.
Sump volume.
Oil return to sump path and mechanism.
Position of oil supply holes in journal.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Main bearing clearances
Is there a way how to calculate it then?
Should I just start calculating everything like bearing surface, force generated by crank, pistons, condrods at redline on bearing surface,...?
But even then I only have force applied on the bearing surface, I still need to know how much force can a certain oil (synthetic that is) take before it collapses at a certain layer thickness.
Or am I way out?
Cheers
Ziga
RE: Main bearing clearances
It is not the hydraulic pressure due to oil pressure that supports the bearings, it is the film strength of the oil. Film strength is not dependant on oil pressure.
Oil pressure is used to control oil flow through the various bearings. This oil flow cools the bearings.
Correct bearing clearance is necessary to maintain pressure to ensure that the last supplied bearing still gets enough oil flow to avoid overheating.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Main bearing clearances
I only asked what's the clearance I should run at a given oil grade.
Different grade oils run in a differenc temp. zone and can support different pressure loadings, and that's what I'm after.
Cheers
Ziga
RE: Main bearing clearances
It's generally accepted that with automotive size crank journals, and in the absence of concrete evidence to the contrary, in an engine intended for performance work, 2 to 2.5 thou on the big ends and 2.5 to 3 thou on the mains is about right. For "normal" use you could close them up half a thou without to many problems. Thrust clearances vary more but around 6 to 8 thou is an accepted figure to shoot for, not being wise to go tighter but I've built a few engines around 12 thou and had no definable problems.
They may seem a little wide, compared to OE, but the increased oil flow through the bearing controls the temps better.
Using lower viscosity oils is no reason to close them up.
John.
"It's not always a case of learning more, but often of forgetting less"