Clearance Crank Pins - Connecting rods

[Manxnorton]

Hello everybody,


I am new to this forum. I am living in Germany and in late 2020 I took over a Model A Roadster from my father. It had not been used for a couple of years. Early last year I had it running again and during spring and summer I improved some details. Unfortunately in October a big problem occured as the crankshaft broke .



I took everything apart and was able to buy a used crankshaft. I decided to purchase a set of new connecting rods with inserts, not the the babbitt style rods.



Now I am at the point to have the crank pins of the crankshaft ground to the right diameter. In the Les Andrews book I found information that the clearance should be 0.001 to 0.0015 inch. From specification tables I found that the difference of the connecting rod eye and the crank pin diameter is 0.001 inch, which confirms the above information.


Now I talked to a person / company who provides crankshaft grinding services. This person told me, that for this kind of old engine the clearance should be 5/100 mm (equal to appr. 0.002 inch). He is an experienced person and I trust he knows what he is talking about. Nevertheless I would like to double-check this information with Model A experts.


- Are there different clearance specifications for babbitt rods and insert rods?
- Is clearance 0.001 inch recommended for babbitt rods only?

- What do you think abot clearance 0.002 inch for the insert style rods?


Thank you very much in advance for your comments and recommendations.


Kind regards
Axel

[Patrick L.]

I think clearance is clearance. I set them at .0015".

[nkaminar]

The clearance is dependent on the diameter of the journals. For a Model A crankshaft it should be 3 to 4 mm.

[old ugly]

the specs seam to be different depending on what book you read.
as i recall ford manual specs are as loose as .003"
i was told by a machinist a while back .0005" for each .500 diameter of the crank pin, + or - .0005". that would = .0015" . +or-
here is the document from snyders when you buy their rods. it says the same as what the machinist told me. https://www.snydersantiqueauto.com/c...201-9-2012.pdf

on the inserts you can not change the clearance like on a shimmed rod. it is all set by the bearing manufacture and the machinist who turned the crank for you.

i just did an engine with new rods and insert bearings. they all plastigauged at .002" and i am comfortable with that amount of clearance.

nkaminar , you must meen .3 to .4 mm but i don't know what that converts to

[J and M Machine]

Quote:- Are there different clearance specifications for babbitt rods and insert rods?
- Is clearance 0.001 inch recommended for babbitt rods only?

- What do you think about clearance 0.002 inch for the insert style rods?

Model A engines have rod and main clearance at .0015" not .001"
The inserts should also go by this rule.

Important part for the grinder to know is the radius of crankshaft has to be maintained. I would also recommend he crack check it before grinding.
http://www.jandm-machine.com/grinding.html

[Bob Bidonde]

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When talking about clearances, you need to consider what they are when the parts are new, and what they are when the parts are worn.
New Bearings
From Dyke's Encyclopedia: Main bearings are line reamed with a 1.625" diameter reamer which is 0.002" larger in diameter than the bearings (newly poured). Shims are added to provide a clearance of 0.002".
Service replacement connecting rod babbitts are reamed to a diameter of 1.409" and shimmed to get 0.002" clearance to the crankshaft. Note that the clearances are the same for the main and connecting rod bearings.
Worn Bearings
The question is, "What bearing clearance triggers an a clearance adjustment?" Answers for this questing are likely to vary widely, so this is my practice. I drive my engines until I hear noise that I suspect are from loose bearings. Or when an engine gets to about 50,000 miles, I adjust the bearings. Or if the oil pan is down for any reason, I check and adjust the bearings. I find the bearing clearances may get to 0.004", so I adjust them to 0.0015" to 0.002".
Bearings That Wear The Most
The center main bearing carries the highest crankshaft loading, so it is a barometer for wear. Connecting rod bearings are highly loaded, so expect them all to need adjustment.
Crankshaft End Play
The clearance for crankshaft end play with new babbitts is 0.004"to 0.007" at the thrust surface of the rear main bearing. The thrust surface of the bearing is principally loaded by operation of the clutch, and thrusts loads from the drive-line which tend to push the crankshaft fore-aft. The thrust bearing surface has a high wear rate and it often cracks away, so it is not unusual see the end of a crankshaft at 0.01" on a high mileage engine. Unfortunately, end play is not adjustable, so new babbitt is the fix. What is the wear tolerance? The amount oil puddle under the car you can tolerate! I would get anxious if the end play of the crankshaft exceeds 0.06".

[JayJay]

Originally Posted by nkaminar

The clearance is dependent on the diameter of the journals. For a Model A crankshaft it should be 3 to 4 mm.

Uh, something wrong here. 3 mm is about 1/8" (0.118"). I think you'll get a bit of knock with that type of clearance...

JayJay

[Bob Bidonde]

When talking about clearances, you need to consider what they are when the parts are new, and what they are when the parts are worn.
New Bearings

From Dyke's Encyclopedia: Main are line reamed with a 1.625" diameter reamer which is 0.002" larger in diameter than the bearings (newly poured). Shims are added to provide a clearance of 0.002".
Service replacement connecting rod babbitts are reamed to a diameter of 1.409" and shimmed to get 0.002" clearance to the crankshaft. Note that the clearances are the same for the main and connecting rod bearings.

Worn Bearings
The question is, "What bearing clearance triggers an a clearance adjustment?" Answers for this questing are likely vary widely, so this is my practice. I drive my engines until I hear noise that I suspect are from loose bearings. Or when an engine gets to about 50,000 miles, I adjust the bearings. Or if the oil pan is down for any reason, I check and adjust the bearings. I find the bearing clearances may get to 0.004", so I adjust them to 0.0015" to 0.002".

Bearings That Wear The Most
The center main bearing carries the highest crankshaft loading, so it is a barometer for wear. Connecting rod bearings are highly loaded, so expect them to need adjustment.
Crankshaft End Play
The clearance for crankshaft end play with new babbitts is 0.004"to 0.007" at the thrust surface of the rear main bearing. The thrust surface of the bearing is principally loaded by operation of the clutch, and thrusts loads from the drive-line which tend to push the crankshaft fore-aft. The thrust bearing surface has a high wear rate and it often cracks away, so it is not unusual see the end of a crankshaft at 0.01" on a high mileage engine. Unfortunately, end play is not adjustable, so new babbitt is the fix. What is the wear tolerance? The amount oil puddle under the car you can tolerate! I would get anxious if the end play of the crankshaft exceeds 0.06".

[katy]

Can the insert rods be used in a Model A engine w/out modifying the oiling system?

[Big hammer]

Yes

[Jim Brierley]

On my Model B race engine I set it at .002"-.0025" on babbitt or inserts. This is a little loose for touring engines but loose is much better than too tight. My street engines are at .0015"-.002".

[nkaminar]

Thank you all for correcting me. Should be 0.03 to 0.04 mm.

I have insert rod bearings and they are scoop lubricated.

I think if you are running pressure lubrication you can run the clearance looser. Plus it depends on what weight oil you are using.

Terry recommends 0.0010 to 0.0035 for his rod bearings but they are 2 inch diameter and pressure lubricated.

"Main and Connecting Rod bearing clearance 0.002 inch is ideal,
0.0010 - 0.0035 inch is acceptable (per the Clevite catalog)"

[Manxnorton]

Thank you very much for all your comments. Obviously there are various opinions about the amount of clearance required. The range mentioned in the various comments includes the value my grinder had mentioned. Thank you once more.

[old ugly]

Originally Posted by katy

Can the insert rods be used in a Model A engine w/out modifying the oiling system?

for the rods the answer to this is yes. it works just fine.

you only have to machine the crankshaft.
but you need to have your crankshaft ground to .010,020,030,or 040 under, one of those specific sizes, exact.
the machine shop needs to know the undersize bearing availability.
then have them tell you what size under they can get out of the crank. once the shop measured and knows what they can turn the crank to then order the bearings.
if you are doing NEW babbit for the crank mains bearings, the mains need to be turned just to true up the journals, for babbit they are not a specific size. the babbit is line bored to fit the crank.

if you are doing main inserts that is a whole different story.

edit
i forgot to mention that this is for dipper style rods only.

[jack backer]

The hydrodynamic principal is clear, .001 per inch of shaft diameter.. for instance a 10 inch diameter journal can use oil as a “cushion” with .010 clearance.

A model A crankshaft journal diameter is 1.5 inches which translates to an .0015 clearance..

[BrianH]

My newly built touring motor (44 miles) has both mains and rods at .0015-.002. Motor sounds real good with that.

[Terry Burtz, Calif]

Nkaminar,

Thanks for your comment.

In the "Builders Guide", I state that 0.002-inch clearance is ideal, and further state that Clevite (a bearing manufacturer) says that 0.0010/0.0035 is acceptable for the inserts that they manufacture.

Clevite's acceptable tolerances are likely based on inaccuracies found in rebuilt engines where the cylinder block bores have been honed and the crankshaft has been ground for an undersize bearing.

The fact that Clevite bearings work with 0.0010/0.0035 clearance is a testament to their quality.

I do not recommend 0.0010/0.0035 inch clearance.

Of all of the "New Engines" being built, I am unaware of any bearing clearance that is not close to 0.002 inches.

[nkaminar]

Terry, Thanks for your input.