Bearing Clearance

[dirtdevil]

Did some internet searches and answers seem to widely varied. Helping a friend with a IMCA modified motor build. Compstar crank 3.75, Compstar rods 6.00. Block is an older - but good - Chevy 4 bolt w/ 4 bolt mains.

What are good clearances for the rods and main bearings? They are standard size and we will be using KING bearings.

Also I have micrometers from back in tech school, but he says Plastigage is usually fine for a build like this.

Thanks for the help.

[perfconn]

Depends on journal sizes.Plastiguage is a last resort.

[dirtdevil]

The crankshaft and rods are new. 350 main jornals and large rod diameters.

[perfconn]

.0025 on rods and. 003 on mains.

[dirtdevil]

.0025 on rods and. 003 on mains.

Thank you and I will use my micrmeter set.

[dirtman45]

The problem is what do you do when you don't end up with .0025 on rods and .003 on mains?

My current build is .002 rods and .0025 mains. You do have a small window of a range to work in. Never go bigger than .003, that would be your top of the range clearance.

The other option is to mismatch the top and bottom of the bearing if you end up with too much clearance. Like a std top and a .001 oversize bottom.

Plastigage is your best option, you get an exact reading. Micrometer quality and how they are used can give you more variation.

Two thumbs up for king bearings.

[istock11m]

I would trust your micrometer. Plastigauge varies do to age and deteriation. Older it is the less accurate it can be.

[racer69]

I will add 2 more thumbs up for Kings! great tolerances. They had me in front of the class at IMIS telling my story about them!

Check everything with the Mics then before final assembly check again with the plastigauge. You usually have to swap around uppers to lowers to get your numbers. Less so with the Kings. They cut the whole set from one sleeve.

[500BOMBER]

In motors that stay under 8000 RPM, 2 to 3 thou. on the rods and mains seem to be a good ballpark.
Above 8000 RPM, I like to get the rod clearance in the 3 to 3 1/2 thou., especially if we run a heavy oil like 20w50 or 25w50. They seem to be less likely to spin a rod bearing that way. Just a preference on my part.

[fast_crew]

In motors that stay under 8000 RPM, 2 to 3 thou. on the rods and mains seem to be a good ballpark.
Above 8000 RPM, I like to get the rod clearance in the 3 to 3 1/2 thou., especially if we run a heavy oil like 20w50 or 25w50. They seem to be less likely to spin a rod bearing that way. Just a preference on my part.

With the proper rods and bearing (proper eccentricity), you should never need more then .0025".

[perfconn]

Too much clearance will cause problems also.You will beat the bearings out.

[oldgold]

bearing clearances

Too much clearance will cause problems also.You will beat the bearings out.

put together my brothers engine with .002 on rods and .0025 on mains ran on engine run test stand for 1.5 hrs good oil pressure good water temp .changed oil and filter and shipped it 5 hrs north for him to put in and it ran 8 laps and spun a bearing. .will a engine with tight clearances and a high volumn pump cause bearing failure. would i have been better off with a standard volumn pump.?happy holidays and where i come from MERRY CHRISTMAS.

[perfconn]

Too tight of clearance will cause bearing failure.High volume pumps will not.If you are pumping the pan dry then its because you are not getting the oil back to the oilpan.

[dirtman45]

You do not want a high volume pump. I've never seen too tight of a clearance spin a bearing.

[let-r-eat]

put together my brothers engine with .002 on rods and .0025 on mains ran on engine run test stand for 1.5 hrs good oil pressure good water temp .changed oil and filter and shipped it 5 hrs north for him to put in and it ran 8 laps and spun a bearing. .will a engine with tight clearances and a high volumn pump cause bearing failure. would i have been better off with a standard volumn pump.?happy holidays and where i come from MERRY CHRISTMAS.

Your high volume pump isn't the culprit.. A high volume pump pumps the same amount of oil as a standard pump, the difference is it's CAPABLE of pumping more if necessary.The problem more than likely wasn't the clearances you used although a little tight but the failure to get oil to the bearing under running conditions. ie pickup too close to pan, oil drainback inefficient, over heat from insufficient cooling, detonation from too much timing, etc. An engine with tight clearances doesn't like to be overheated.Someone a whole lot smarter than me showed me something that stuck with me. He took a crank and warmed it to 200*. Took the rod and heated it to 200*.....Then measure clearances.....You will quickly see that things don't stay where even the best machinists put the tolerances. A whole bunch of this (not a nice word)(not a nice word)(not a nice word)(not a nice word) you read is hockus pockus to sell parts. I've ran cranks at 10 thousands *of course this isn't recommended* clearance with no ill effects. Looser will always be better than tight as long as you can maintain required pressure to lubricate. Too lose requires HP to move the oil and require oil pumps capable of the task.