B P Blowout Leak Update

Bill Edstrom

Bill Edstrom

Member
I will review the problem that I posted a couple weeks ago. I was on a trip with the Up North Club in Wisconsin and at about 700 miles on a rebuilt engine, the engine stopped at an intersection. We jiggled the wiring and the car started. We had about a 30 mile drive to our lunch stop and on the way I turned on the lights and the engine would cut out, which indicated a loose connection. Also along the way I stepped on the brakes to slow down and the engine back fired. This is the one and only back fire I had in the 700 miles and none since. When we got to our lunch stop I found the loose connection at the ammeter. The
terminals on the ammeter were loose. At this stop I noticed oil flooding out of the flywheel housing. I checked the oil and it was a quart low—so I added a quart. We drove about 40 miles and I checked the oil again and it was a quart low. Oil was still flooding out of the flywheel housing. We added another quart and we drove a hundred miles home and there was no loss of oil. So I dropped the pan and removed the rear main cap to check it.
1. Photo #1 shows the rear main cap and notice it is in very good shape. The green line is plastigage that I am using to check the clearance.
2. Photo #2 shows the crankshaft with the plastigage in place after the rear main cap nuts have been torqued to 80 foot pounds. The plastigage shows about .0015 to .002 clearance tapering to .003 at the very rear near the crank slinger.
3. Photo #3 shows the plastigage clearance after removal of shims to
reduce the clearance by .0015. I was smart enough by now to stick the plastigage measuring scale in place. The gage indicates that we now have a clearance of .001. There is still a slight taper toward the crank slinger. So with this
clearance I installed the rear main cap.
4. Photo #4. Here is the Model A I am talking about. Please note the lady in the photo with me. She flies airplanes, drives Model As and has put up with me for 56 years.
CONCLULSION
I have driven the car over 200 miles and have not had a drop of oil from the flywheel housing. The car is a sweet running Model A. It runs very smooth at 55 miles per hour out of overdrive and even better in overdrive. I have a Mitchell Overdrive and I have not had the car to 60 MPH yet. As for the oil leak, I had to be getting flow by the cylinder and pressurizing the crankcase some way causing oil to be forced out the rear main. I can’t explain it but I feel it is related to the loose connection at the ammeter and the single back fire.
I would appreciate any comments as to the cause of this problem. It is a mystery to me.
 

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Bill, several things are factoring in. Your .001" reading may not be that exact if you did not factor in the weight of the crankshaft since the last means of support would be at the center main. That is why it is recommended to push upward on the crankshaft to firmly seat the crankshaft pin journal in the babbitted bearing in the block when taking a reading.

Next, the taper would have me concerned as it can allow oil to escape. It may take a few repeated attempts to "burnish" the babbitt enough that it will conform to that taper and provide a uniform clearance. Nothing to be overly concerned with IMO but it may take a few attempts at re-clearancing.

Finally, one thing I noticed is it appears the shims in one of the picture is possibly torn and coated with something. Make sure the oil is not seeping past those shims and causing your leak.

Consult with your engine builder and advise him of this issue and see what he says.

.
 
Also, make sure the oil tube is not installed to far in to the cap and is clear of any obstructions. If you are using the old style aluminum upper seal, maybe it is loose and oil is getting past it.
 
Bill Edstrom said:
.......... Please note the lady in the photo with me. She flies airplanes, drives Model As and has put up with me for 56 years..........I would appreciate any comments.................
Click to expand...

My considered comment is forget working on the car and spend as much time as possible with your lovely lady...Priorities, man, look to your priorities. JMHO.
 
Brent

Thanks for your comments. Plastigage is really more of a good estimate of clearance than being exact and you are ridht the weight of the crank needs to be considered. The builder Joe Thoren came up and inspected the engine after I removed the main bearing cap and he does not have an answer as to why the rings could become unseated after 700 miles. Anyway the results so far after tightening the clearance has been excellent. I removed the inspection cover from the flywheel housing yersterday and there is just not any oil leakage from the rear main. Now if it remains like this for the next several thousand miles it will be great. The car is really fun to drive and the fall here in Northern Wisconsin is great and we will take advantage of it till snow comes.

Thanks to all that have comments you are all very generous.

Bill eEdstrom
 
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