Connecting rod clearances, what do you think?

O

old31

Well-known member
Using plastigauge, My connecting rod clearances a .002 through .003, what do you think?

If not good, what is the fix?
 
Not good. The fix, reduce the clearance.

Measure shim thickness.
Peel shims and try to make each side the same thickness.
Remove a shim, install cap with plastigauge, remove and take a look.
Do each one until they are right. I prefer .0015"
 
Patrick L. said:
Not good. The fix, reduce the clearance.

Measure shim thickness.
Peel shims and try to make each side the same thickness.
Remove a shim, install cap with plastigauge, remove and take a look.
Do each one until they are right. I prefer .0015"
Click to expand...

My method is to take out all of the shims for each con rod in turn and measure their thickness. I then separate them and lay them out in order of thickness and with some simple arithmetic, I remove say, 0.003" from the thicker side to reduce the clearance by 0.0015". I have some spare shims that I use to get things right if there is no 0.003" shim in the rod. Reassemble and check the clearance.
It should be right but be prepared to swap a shim or two about in the pack. For example, if you need to remove 0.003" and there is no shim that thickness, I remove say, a 0.010 shim and replace it with one 0.007" thick.
A little addition and subtraction is as much as it takes. Oh, and a little patience.
Aim for a clearance of 0.0015" as Patrick says.
 
Synchro has the right approach. It will not hurt to have one shim pack on one side slightly thinner than the other side. I take a shim out on one side then the other, going back and forth, until I have it right. Taking a 0.002 shim out of one side will reduce the clearance by 0.001.
 
None of my connecting rods had any shims when i took them apart. I used the plastigauge on what I had and that's why they measured .002-.003.

So, do my caps have to be filed down?
 
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With the clearances you have you can still drive many miles before having to redo things if you drive your car as a hobby and not planning to do long distance travel. They will let you know when it is time to replace.
 
J Franklin said:
With the clearances you have you can still drive many miles before having to redo things if you drive your car as a hobby and not planning to do long distance travel. They will let you know when it is time to replace.
Click to expand...

I agree. There is no point over thinking things like this. Back in the day, Model As had a reputation for keeping going way past when they should have stopped. They would rattle like a fistful of stones in a can, yet they kept going.
With those clearances, put it together and drive away.
 
Connecting rod bearings have the highest loading, so take care to get their adjusts correct. If you have the rods accessible, then reduce the clearance to the minimum 0.001" to 0.0015".

Make sure there is no oil in a bearing when checking its clearance.

Check for roundness by placing Plastigauge strips at 4, 6 & 8 O'clock under the caps. Ideally the strips will indicate the same clearance at each bearing, so no out-of-round. I would expect on each bearing, the strips to indicate within 0.0005" to 0.001" of each other.

I find out-of-roundness is in the babbitt, and typically not in the crankshaft journals. To get the babbitts true, use a combination of carving them and varying the shims. Use machinists dye on the babbits to find the high spots that need carving.
 
.002" is OK, .003" is a bit much. You can carefully file the caps or my preference is to use a piece of emery paper on a very flat surface and rub the cap in a figure 8 motion. Sure, it'd run as-is but you might as well do it now so you won't worry about it. Go slow so you don't take off too much.
 
I took a few pictures to illustrate what Jim is talking about. I happen to use my drill press table that is in good shape. If you don't have a flat machined surface a decent sized piece of 1/4" glass will work. I don't bother wit figure 8 just care fully with even pressure rub bank and fort. Flip end for end after a few rubs. You can always paint with dykem to show if you sanding evenly. This cap and set of rods is from a 47k original unrestored model A. No shims on the rods. Bob mentioned cranks they do wear and can be out of round. As long as you can get the clearance to around .002 or less without binding you are good to go. I can't seem to find a way to post pictures?
 
Bob Bidonde said:
Connecting rod bearings have the highest loading, so take care to get their adjusts correct. If you have the rods accessible, then reduce the clearance to the minimum 0.001" to 0.0015".

Make sure there is no oil in a bearing when checking its clearance.

Check for roundness by placing Plastigauge strips at 4, 6 & 8 O'clock under the caps. Ideally the strips will indicate the same clearance at each bearing, so no out-of-round. I would expect on each bearing, the strips to indicate within 0.0005" to 0.001" of each other.

I find out-of-roundness is in the babbitt, and typically not in the crankshaft journals. To get the babbitts true, use a combination of carving them and varying the shims. Use machinists dye on the babbits to find the high spots that need carving.
Click to expand...

I agree,

Just take a little off the caps, not a big job beings it is apart.

My opinion,

Chris W.
 
My experience with my original Model A Engine a case in point.

The odometer of the car read 147K miles. The engine was red, and was likely a well worn Allstate replacement engine. (Not marked) I estimated 70K on that engine - not inconceivable.

My original "tightening" of the engine was done on my back and using plastigage and a jack as I have outlined in another thread. This done with the rods all disconnected and pulled up out of interference.

All of the shims were out of the crank and I resorted to "filing the caps" to bring them closer. As left, everyone was tightened to 0.001, which while challenging, was not impossible to accomplish.

Doing the rods was next and done with the mains "loose" (i.e. the bolts holding it together loose by a couple of turns on each.) I didn't want to be challenged by a "tight" crank.

At least part of the problem with this engine was that the crank rod bearings were worn "egg shaped" - which makes sense when one considers that one side always gets the greater thrust of the piston/rod. The egg was about 0.003. And like the crank, no shims.

Thus it was not possible to "really" tighten those rods. When socked up "to the tight spot," the crank would bind in four bearings twice at each rotation. Selectively, I backed off and made "paper" shims to go in the rod caps. Again, a little selective cap filing to "overshoot" and again not overly tighten the rod over the "egg."

I ran it like this for some time. The engine at low to medium speed would have a sort of "whuppa-whuppa" sound to it, a sound I became conditioned to and to be monitored as a pre-failure indicator. Whuppa-whuppa became a good thing. While I had that, I was still moving and it was a speed point to be above, or below.

That is until the day it didn't, and the whuppa-whuppa became a "banga-banga."

I was driving the Mid-Cape (Cod) highway in the typical weekend busy traffic when it let go. I coasted to the side and then slowly motored in the break-down lane to the next available exit - and then "babied" it home over the Cape back-roads. I was not yet sure of what caused the banga-banga, but I could control it and still motor-vate. So motor-vate I did.

Disassembly of the engine revealed one of the rod nuts had broken its cotter-pin, and unwound to disassembly. I found the nut in the bottom of the oil pan. And the rod cap flapping, but still in place held by the empty stud and the now loose other nut.

At that point I decided to "retire" the engine and replace it with an engine reconditioned with the help of Richard Knight of this board - which is the engine I still use.

BUT - what precipitated that breaking of the cotter and the unwinding of one nut was the periodic vibration created in "incipient binding" of the egg journal in the filed caps.

So, an engine not driven to destruction - although that possibly was not far away.

Interestingly, the red engine I still have and it has an unmolested number pad - with no number. I think I may keep it this way - and repaint the engine red on rebuild.

Some things are interesting in their origins and all by themselves.

And except for judging, how can one go wrong with a red engine? Red ALWAYS enhances the speed, or at least the perception thereof.

Joe K
 
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I would lap the caps. A thick piece of plate glass and a few sheets of 400 grit with some ATF or low viscosity hydraulic oil as a lubricant. The major caution is that the only way to tell how much is removed it to have either a gauge block and a micrometer or a height gauge to work with the glass plate. It's not without risk of needing shims and it also is affecting the service life of the connecting rod assembly. With no shims found there, it may have already been lapped before.
 
My problem is I bought a 30 A and dropped the pan to clean it out and check clearance of the rods and mains.

There are no shims in the first 2 rods I have taken apart. I plastiguaged them and they seem to be within tolerance. Why do I not have shims. I have rebuilt A engines before and they all had shims?

Were there engines with no shims or have all been removed to get proper clearances?
 
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