Triumph Repair: Connecting Rod Bearings, connecting rod bearings, auto machine

Question
I want to replace the connecting rod bearings and mains in my 1970 Spitfire.  I've removed one bearing but can't find any markings on the bearing indicating STD or oversize.  I don't have the bearing-crank clearance.  How can I determine the correct bearing?

Answer
Hi Tom,
The bearing size is stamped into the shell on one end of each shell. There will be an emblem or the name of the bearing manufacture there also. Use a magnifying glass to look closely. You need to examine the mains and the rods because they are not necessary the same size. If you find the manufactures name etc but no "Std" or size then you need to just mike the crank mains and rods (which needs to be done anyway to be sure the shaft is not out of round)

If your engine has not been changed it is a 1300 engine and standard mains if standard should be 2.002" and the crank pins should be 1.625". They can be as much as one half thousandth larger from the factory.

You need to mike the shaft in two directions to be sure it is not out of round. If you don't have mikes, you need to take the crank to a local auto machine shop and ask them to mike it.

I found that you can not count on new bearings being correct so even after the crank is miked and found to be ok, you should use "Plasti-Gauge"(green), to check the oil clearance when the bearings are installed. I like to see one and one half thousandths +/- one half thousandths on the rods and 2 thousandths +/- one half thousandths on the mains. (The auto machine shop should be able to supply you with the Plasti-Gauge (green))

Be sure to lube the bearings when installing. I like to use a 50/50 mix of STP and engine oil. Ask your auto machine shop what they use.

Howard