Center Main Thrust

We got bit again by an engine failure at the May 07 Bugorama. Having the drive tang break off of the oil pump is relatively minor, but the resulting lack of oil supply to the bearings still damaged both the bearings and the case and worse, took us out of competition for that event, dropping us from third place in the standings to seventh.

The main challenge with a case set up for type 4 mains and a flanged crank is prepping a thrust bearing every time. It’s a very tedious process that takes me about four hours to do. The bearings can be purchased, but they’re not always available. With that particular bearing being made out of aluminum, it can’t seem to take the abuse of a momentary loss of oil pressure. Failure seems to be instant and when it does fail, it sticks to the crank and hurts the case. The rest of the main bearings looked fine, but that thrust was toast! Fortunately, it didn’t hurt the crank. It polished up nicely and mag checked OK.

Pat Downs of CB Performance as well as others advised me of an improved set-up that uses BMW tri-metal bearing shells with the thrust at the center main. This set-up uses the stock type 1 main bearing bore size and requires that the case be machined for snap in spacers which take up the gap between the case and the BMW thrust bearing. With these bearings, the thrust bearing needs to be narrowed slightly to work with the Scat flanged crank. (Photo 1) Since that time (Summer, 2007) I have converted over to separate thrust inserts which were made fom bronze bearing stock. After a time, the "Steel on Steel" arrangement of the BMW thrust bearing had beat up the thrust surface on the crank. This was unacceptable, and I decided to make bronze thrust inserts. Photo 2 shows the new arrangment.

The center main boss in the case is counterbored to accept thrust bearing spacer inserts, then the bronze inserts are munufactured. The center main has a ground surface suitable for a thrust either by design or a result of the method of manufacture. Whatever the method, the width seems to be consistent from crank to crank. The other modification is to cut bearing tang slots into the main bore in order to locate the bearings.

This set-up uses a conventional snap in bearing shell much like the rod bearings or virtually every other auto engine. Made for a BMW, they are of a tri-metal design. Steel shell, copper middle layer and a soft “babbit like” material on the surface. Both bearings and case will require slight modifications. A tang slot is cut into the case to locate the bearing fore and aft (Red circle). Dowels are no longer used. No more pinched bearings because the dowels and dowel hole in the bearings aren’t lined up, except for the “nose bearing.” This will still be the stock full circle bearing. The modifications to the bearings will be a slight narrowing and carefully locating and opening an oil hole to line up with the oil holes in the case (green circle in photo 3).

Assembly differs only slightly from normal procedure. Connecting rods are still installed first, using either a micrometer or a stretch gage to measure rod bolt stretch. The crank timing gear can be installed without a full circle bearing installed onto the journal under it. Once the crank is layed into the case, the thrust can be checked with a feeler gage before the case halves are put together as well as checking with a dial indicator after assembly.

With the shortblock assembled, everything else goes back together in typical fashion. It's certainly a solid looking foundation. I was pretty impressed upon final assembly. The crank turns very smoothly.

It does have many logical advantages. The most obvious is the use of high quality main bearings with the exception of the nose bearing. Another is that with the thrust in the center, both sides of the thrust will be virtually awash in oil from crankcase windage, which can only aid in thrust bearing life. Honestly tho, I have only experienced thrust bearing trouble twice and both times were due to oil starvation or improper preparation. This most recent failure was oil starvation. The previous incident was a failure to chamfer the inside of the thrust bearing to allow for a larger radius between the main journal and cheek of the crank. The “grabby” aluminum bearing, just by touching that radius, overheated at the spot, galled and then grabbed, seizing the motor. Lesson learned.

The downsides are few, but substantial. The inability to bore the mains oversize in the event of a main bearing failure and at this time, the cost. The case modification is pretty expensive. The bronze inserts are only made by a few people. I am in the position to make my own, but it's not for the average enthusiast. It is time consuming tedious work.

Time on the motor since the conversion proves how durable it is. I tore the motor down recently after over 100 runs, and both the BMW bearings and the bronze thrusts were in perfect condition.

I do have this to add. With a $1200 crank and several hundred dollars worth of modifications plus bronze stock, it is obviously a major committment. Perhaps for most enthusiasts, it would be wiser to treat the crank as a consumable, and simply use a readily available type 1 main crank and simply replace it after a set number of runs.

Also with this engine edition is the use of the Autocraft “Stage and a Half” oil pump. (Photo 4) It seems that with every new engine, there is some area that has given us trouble, and rather than back-step and use stuff that worked in the past, we “upgrade.” This is the case here. Since it was the oil pump failed the last time out (a “Bigger and Better” version itself), we’re going “More Bigger and More Better” again. It’s certainly much more complex.

The primary pump segment does NOT use is the internal oil pick-up arrangement. It is an external pick-up configuration. Some use a “Sump Plate” pick-up from Autocraft. I have elected to use pick-ups tapped into the side of the sump that have debris screens incorporated into them. (Photo 5 &6) Since we have dragged the sump on the ground during the launch on several occasions, I felt this would be safer. The external pick-up arrangement does not suffer from air leaks and housing gaps like the stock pump does. Correctly built, it is a solid delivery system. I can say that there is more money in the fittings and pick-ups that what the actual pump! However after several hundred runs and several bearing inspections, it is a proven combo. The bearings have been perfect EVERY inspection time.

Photo 1 above, The center main bearing with inserts


Photo 2 shows the "new and Improved" center thrust inserts.

Photo 3 shows the how the oil hole needs to be modified to match the case, and the tang slot that needs to be filed

Photo 4 shows the "Stage & a Half" set-up mocked into position

Photo 5 - the in-sump oil pick-ups

Photo 6 - The suction side. The pressure side is fairly typical of most systems in use.