Technical Big end Bearing Shells

[ronstox]

I've just measured the Big End bearing surfaces on my 110F 499cc engine with simple shop calipers. I get 44.0mm all around, so no measurable ovality, in both bearings.
I could detect no looseness or play when I pulled the conrods off, but maybe ill-fitting bearings are not detectable by mere mortals.
When I look at the specs in Haynes, I read:
Standard: 44.013-44.033
0.2mm undersize: 43.759-43.779

Ok, my calipers can't read to 5 significant figures, but it's clear enough that I am falling somewhere between the standard and the 0.2mm undersize.
So what size shells should I order?

 

[Goldnrust]

For measurements of this accuracy, often callipers just aren’t accurate enough. Especially when doing internal diameters.

If you don’t have access to micrometers and bore gauges, then plasti-gauge is a cheap alternative for measuring clearences. It won’t work on the main bearings of a 500, due to the full circle bearing design, but it will work for your big ends.

If you do fall between the two bearing sizes, you have two options. 1 put it back together with stock bearings, knowing the crank is a little worn and there is a risk it might make for low oil pressure and could wear more in use. 2 have a machine shop grind the crank to the correct 0.2mm underside measurement and buy the undersize bearings.

 

[ronstox]

Thanks Goldenrust.
Haynes says that the Big End clearances when new are .011-.061mm, with a wear limit of 0.15mm.
So, applying this limit on the standard bearing gives us a conservative wear limit of 43.88.
So, with my sad little caliper, which probably is accurate to +/- 0.1, my bearing measurement is 44.1-43.9.
Still inside the wear limit. I'll choose option #1 and put it back together with stock bearings.

 

[Bounding Bambino]

If your crank journals look good and have no scoring. I would suggest stay with std size and go for it!

If you have a oil pressure gauge, or have an inclination to install an oil pressure gauge that would be ideal. Ive seen mine typically startup is around 60psi cold (seems to be standard pressure relief pressure). and then once up to full temperature after a spirited drive at idle (~180f) pressures hang around 25-30psi with pressure instantly shooting up to 55-60 once engine is revved which is a very healthy target to aim at for a healthy rebuild, and good clearances to maintain pressure thru out the system.

 

[ronstox]

Thanks for that info, Jacques. I have been thinking about setting up an oil pressure, temp, & rpm system. It has been in my 'nice to have' pile, but maybe now it's the right time to get that together. I like LazyLobster's set-up so i might try and copy that.