After installing new valves and adjustable lifters my valve noise is gone, but now I'm able to hear what I think is main bearing knock at 30 MPH that diminishes if I retard the spark. Are there any tricks to checking the clearances, and is plastigauge OK as I'm concerned with the weight of the crank giving a false reading. Also, can I get the rear main cap off with the engine still in the car ( it's a'25 with the four dip cover)? I am using a Z head ,and wondering if I have too much compression as all four cylinders have just over 60 pounds at cranking speed. Thanks : Bruce
I too have some familiarity with engine sounds. My limited experience tells me that there are two distinct "knocking" sounds that come from T engines. The main bearing sound to me is a very deep thud like sound. The cam bearing sound is more high pitched and reacts to the change in timing, "retarding". Just my .02. Dick C.
60 PSI of cold cranking pressure (this is not compression) is pretty low for a Z head. It indicates that either the new valves are not sealing too good or the ring / cylinder wall relationship is tired. It is not too much compression by any means, it is about what we see with a freshly rebuilt T engine with a standard Ford iron head.
You can get #3 main off with a 4 dip pan. I don't think Plastigage is much use on a tired engine because the bearings do not wear symmetrically. You put Plastigage in one spot, it only tells clearance in that spot. Not much to be learned that way.
The Ford manual tells how to adjust main or rod clearance. I would try removing one shim and see if it is then acceptable.
Thanks Royce, if I remove shims one at a time, and can still turn the engine with the hand crank, am I OK? I've never had the main caps off, but the rods appear to have been done recently and all are at 0.0015" (with plastigauge).Bruce
Sometimes I remove one shim and the engine can be turned with the crank or electric starter, but it is too hard to start that way. I then have to get a buddy to tow me around the block once to wear in the babbit so the car can be hand cranked.
Thanks again Royce, I would have expected the babbit to melt if it were that tight. It's tougher than I thought. Bruce
The mains are line bored. Be careful that you do not distort the crankshaft removing shims. If the center main is too loose, it will make a thump noise when the engine is under load.
Royce : What should I expect to get for cold cranking pressure with a Z head, assuming the valves and rings are near perfect? Thanks : Bruce
I think any time you remove a shim the crank will be forced up. So it will bow the crank a bit. It's inevitable. You either do that or pull the engine and rework all the Babbitt.
Remember, the bearings wear egg shaped. It is simply what happens. You and I will (I already have) break cranks one day. After that we replace whatever is broken.
I measured 70 PSI on a used engine that was in good shape with perhaps 20,000 or more miles on it with a Z head. Maybe someone else has results on a new engine?
Thanks Royce; Having said that, do you, or should I have any concerns about damaging the mains with this much pressure? I sure like the performance gain with the Z head, but am now considering putting the stock head (milled .060) back on just to add some more lifetime to the engine. BTW, for comparison, what is the cranking pressure with a stock head? Thanks again : Bruce
One knock which happens with a very loose engine is when the pistons rock back and forth in the cylinders. When you have the head off, can you move the pistons back and forth by hand? If so, this can happen while the engine is running. It is most noticeable with the spark advanced.
A stock Model T freshly rebuilt has 55 - 60 PSI cold cranking pressure. If you are in Denver I would expect 50 - 55 PSI due to altitude difference in air density.