AMC 401 bearing clearance

dwg86 wrote: I am assembling my 401. I plastic gauged the mains and I'm getting .001 clearance on 1-4 and .0015 on number 5. Seams tight to me. This engine is getting rebuilt because #8 piston and cylinder wall was scored, and it sounded like it had a rod knock. Oil pressure was 65 psi hot running down the road, 35 hot at idle 10w30 oil. The bearings looked trashed when I tore it down, but crank looked good. I know the service manual says .001-.002 main clearance, but .001 sounds tight. Opinions or real world experience have any advice?

I spent three years tracking down a low oil pressure problem with my AMX's 401 after having it rebuild. Turns out the machine shop used the clearance specs provided by the bearing manufacturer which was twice the clearance they should have been. They measured between 0.0035" 0.0040". It flowed too much past the main bearings so it didn't hold enough back for the cam bearings and wiped all five.

The last time it was done clearances were set to 0.0017" (spec is 0.001" - 0.002"). It holds good pressure and was enough clearance to comfort the machinist.

These things aren't Chevys so don't treat them like they are. Stick with AMC's specs from the factory service manual.

Don't use Plastigauge, it's not accurate. Use a micrometer and bore gauge.

-Steve-

6PakBee wrote: Steve, I've used Plastigauge in the past with what I thought was success.  What kind of discrepancies have you seen?  I will admit that when you are at the end of the 'range' of a given size (either high or low), reading it may be something like reading tea leaves.

I used Plastigauge early in my diagnosis of the low oil pressure problem I was having. The clearances measured within AMC's specs. You can see this journal measures about 0.0015" using Plastiguage.



When measured with a micrometer and dial bore gauge the measurement was much different, almost 0.0040". Possibly the other side of the journal still had space between it and the bearing.

-Steve-

.0025 on the rods and mains will work well in a 401 performance application. 

To set are bore gauges we use gauge blocks to set the mic to size wanted in tenths and lock mic. Then using the locked mic we Dial in bore gauge that also reads in tenths to the mic. For acurate measurement you always want your measuring tool one scale better then you want in output. You want to measure .oo1 you need measureing equipment capable of .0001.

Edited by 53w - Sep/26/2016 at 7:55pm

For an inexpensive gauge you can use the one in the link below and set it up with micrometers.