At the VSCDA Elkhart Lake Vintage Festival in September the oil pressure
dropped from the normal of 50 to 25. I went straight into the paddock and
when we dropped the pan we found that the center main bearing was pretty
well gone, no copper left and pits in the third layer. A lot of large (1/4
inch square, about) pieces of copper debris in the pan. These were the King
County Tri-metal bearings. Now that I have the crank out I see that the
rear main is just about as new and the front main is just starting to show
some copper. The crank has been cracked checked with a "sonic flow" process
and no cracks were found. It also checks out straight according to the
machine shop. We run a usual 5500 redline with occasion up to 6000. It is
a stock crank that has been balanced. The rods are stock (look to have been
balanced) and we have the K. Gilanders 87mm sleeves and forged Wiseco
pistons. The C.R. is about 11:1 and a Kastner G3 cam that has been reground
at one time but degrees out to the G3 spec. Cylinder head is just about
stock except for cleaning out the rust and slag from the ports when we had
it milled and new guides and stock valves put in. This was only the 5th or
6th weekend on the bearings and reground crank, 0.010 under. The rod
bearings are not showing very much wear at all. We have the finned aluminum
oil pan which holds 8-1/2 quarts from empty. We use Valvoline 20W-50 VR1
racing oil. The water temps and oil temps are always pretty good, if
anything a little cool. We have a well ducted aluminum radiator and oil
cooler.
I've contacted BPNW, where I got the bearings, and will send them to them
and they will forward them to King. They have generously replaced the
bearing set without my asking! A great place.
Question - any ideas why a catastrophic failure of just the center main
bearing?
2nd Question - Have to grind the crank to 0.020 under on the main journals
now, what clearance is appropriate?
3rd Question - What has been the experience with those who have had the
bearings coated? I've heard some good reports and no bad reports so far.
Any recommended sources?
In going back and checking the numbers from the last crank grind I now think
I only had 0.001 to 0.002 clearance, maybe 0.002 to 0.0025 at the most. The
bearing with the least wear, the rear, had about the same clearance as the
center, from my notes on the measurements. The front bearing, which is just
starting to show some wear, probably had the most clearance.
Last Question, just curious - does anyone know what the "draft angle" is on
the block for the main bearing caps to seat in? If I just set the bearing
caps down in the block I get about 0.001 to 0.002 clearance to the block,
until I tighten the bolts then there is essentially no measurable clearance.
Final note: the machine shop was suggesting I have the block line bored,
when we thought we had a cracked crank. It would seem that there is no
damage to the block and it is in the same state it was before, very well
stress relieved after about 50 years!
Thanks for the help.
Tim Murphy
1961 TR4 #317
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