Fitting Rod Bearings

U

ursus

Well-known member
I am working with new rods on a reground crankshaft and wondering about something I read in the old Victor Page book on Model A mechanics. He states that a rod and piston assembly should be able to fall by its own weight from a vertical position. That seems too tight as a rod fitted using the 0.002 foil method seems to fall by its own weight without the additional weight of an attached piston.

Am I OK with a rod that will fall from vertical by its own weight?
 
Yes, that will work fine. I agree with you...any rod that requires more weight than its own is too tight for me.
 
With precision machining of the rod bearings then the rods should fall without the pistons. With hand scraped bearings in the past, they were not all that accurate and were fitted tight. They were carefully broken in so that the high spots were worn down. The Model T engine was assembled very tight and smoked in on the assembly line using an electric motor. The engines were upside down without the pan on and a worker would stand over the engine pouring oil on the bearings while smoke was pouring out of them.
 
nkaminar said:
With precision machining of the rod bearings then the rods should fall without the pistons. With hand scraped bearings in the past, they were not all that accurate and were fitted tight. They were carefully broken in so that the high spots were worn down. The Model T engine was assembled very tight and smoked in on the assembly line using an electric motor. The engines were upside down without the pan on and a worker would stand over the engine pouring oil on the bearings while smoke was pouring out of them.
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I went back over the details of Victor Page's section on engine assembly and now understand the point made by nkaminar. There was a lot of bearing scraping back in those days.
 
How much clearance do you measure between the crank shaft and the rod bore??
I would suggest .0015"/.0020" total clearance. If you don't have a good way to measure, plasti-gauge is better than nothing.

Or- install the rod on the crank pin with assembly lube or motor oil, torque to 50 ft-lbs. If no drag at all, start peeling shims evenly from both sides until you feel moderate resistance turning the rod. Add one shim back to one side only and you should have just the right amount of clearance. This is based on shim layers of about .002/.003" thick.
 
Dan McEachern said:
If you don't have a good way to measure, plasti-gauge is better than nothing.
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Just for reference, what is the preferred way to measure this clearance? I assume you mic the OD of the crank and then tighten the rod without the crank in it and use a bore gauge to measure the ID of the rod?
 
alexiskai said:
Just for reference, what is the preferred way to measure this clearance? I assume you mic the OD of the crank and then tighten the rod without the crank in it and use a bore gauge to measure the ID of the rod?
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That is correct. Measure several places on the crank journal looking for size,taper and roundness. Same with the rod after torquing it to spec.
 
How does one get the leafs off the rod shim stack? Hopefully without making a mess of the stack. I have am engine with not all that many miles on it. I was working on the carb, and heard the faintist of noise when I would advanacve the throttle about 10%, and I would hear something upon deceleration. That is how faint it is. Then I noticed it is getting a little more easy to hear. I took straps off the spark plugs until I isolated the rod number. Now I have the pan down, and want to take the rod up just a LITTLE.

When I put my micrometer on the shim it came out to read 34. (one turn around is 25, plus 9). I can not see the leafs with the naked eye. As I recall they are about as thick as a piece of notebook paper.

There must be an easy way.....
 
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I've used a Bic lighter to heat a corner then use an Exacto knife to separate the laminations - hard to get just one shim !
 
Most laminations are in the .002-.003" range. A sharp box knife or exacto knife blade is a really good tool to use to peel laminations with starting from one end or corner.
 
I barrowed my wifes desktop magnifier with LED lights in it, and a new single sided razor blade. Checked each layer I pealed off with a mike to make sure of thickness removed.
 
Three thousandths of an inch - holy smokes!!!!!!!!!!!! Fine and dandy now. Guess it doesn't take much.
 
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