Join Date: Dec 2012
Location: Gastonia, NC - Born & raised in Connecticut - 31 years
Mentioned: 1 Post(s)
Tagged: 1 Thread(s)
Quoted: 388 Post(s)
Here is a tip you may want to look into. I don't know the history of your build, but, it is very important to get the correct "crush" on the bearings. You need some tools to do this, or have a machine shop do this for you and you can sleep at night.
Torque the main caps down, without the bearings. Measure the inside of each bore. My info states that the bore is measured at the up and down 12 and 6 o'clock position. The bore for the 400CI 3" mains should measure 3.1880 to 3.1885. 3.25" mains should measure 3.3480 to 3.3485. If you do not get the correct bearing crush on the main bearings, then you will not get a true readings from your bearings. Best way, as mentioned in an earlier reply, is to then insert your bearings, torque down the main caps, and measure the bearings while in place at the 12 ans 6 o'clock position. Use a micrometer on each main journal and measure these, writing them down. Subtract the main crank diameter from the inside bearing diameter to get the true clearances. You may have some variances and you can switch bearings around to get the best placement of your bearings. This can be done with the rods as well.
If your crank has been cut, make sure your bearings have a large enough chamfer on the edge to crank radius or you can destroy your bearings.
Factory crankshaft end play is listed as between .0030 to .0090. If not in this range, then you may need a different thrust bearing.
Check the clearance between 2 rods with a feeler gage. My info states that the factory manual is .006" to .011". However, the book I have for building a street Pontiac states .014" Clearances here are important as too much will cause oil pressure loss and overload the cyl walls and overload the oil scraper ring on the piston.
My Pontiac build up book states main bearing clearance as: Street- .0005 to .002, Ram Air/Super Duty- .0015 to .0024, Drags - .0025 to .0032.
Rod clearance as listed as: Street- .0005 to .001, RA/SD- .0013 to .0033, Drags- .0020 to .0035
My Chilton Pontiac factory engine specs, 1968 to 1973 GTO/Lemans/Tempest calls for:
Main bearing clearance @ .0002 to .0017
Rod bearing clearance @ .0005 to .0026
1968/1969Rod side clearance .006 to .011
1970 to 1973 rod side clearance .012 to .017
1969 RA IV mains 1,2,3,4 @ .0012 to .0028 #5 main .0007 to .0022
rods @ .0015 to .0031
1970 RA IV mains 1,2,3,4 @ .0007 to .0023 #5 main .0007 to .0022
rods @ .0015 to .0031
Personally, I have never done this on any engine I built as I have simply "assumed" my machinist knew what he was doing when he turned down the crank and told me what bearing to order. I have spot checked with the plastigage, but never gone all out. My last 400CI had oil pressure problems and it eventually dropped to "0" and don't know if my "assuming" was part of that problem. You can bet this time around I will take the time to personally check each and every tolerance this time around. Engines today cost way too much to guess or "assume" and I can't just go to my local junkyard and buy a good running 400CI for $150.00 anymore! HahaHa
Hope this helps.