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norcalal 05-21-2015 11:52 PM

shimming engine bearings
 

Model a engine had a rattle starting with just medium throttle in second gear. Center main was over .003 then, brought it to.015, spun fine, I took a shim out of one side of the rear main, now extremely hard to crank, but starter will turn it freely. PlastIguage shows .015. don't want to burn up a bearing, should I reassemble with too much clearance according to specs? Please advise!! thank you

Mike V. Florida 05-22-2015 12:24 AM

Re: shimming engine bearings
 

You should be fine as it is, the spec is .001 if i remember correctly.

Patrick L. 05-22-2015 05:08 AM

Re: shimming engine bearings
 

.0015" should be fine. I'm wondering though why it is turning hard. It is turning harder than before the adjustment ? Did you measure the shim packs before removing any ? You didn't mention anything about them so I'll assume the bearings looked good ? I'm also wondering if maybe one of the shims is contacting the crank somehow.

Kohnke Rebabbitting 05-22-2015 10:24 AM

Re: shimming engine bearings
 

.001-50 Minimum, or plus .000-50 Maximum, which is a .001 per inch of crank diameter.

.001 thousandths was used when the engine was run in cold under another power source.

Kohnke Rebabbitting 05-22-2015 10:30 AM

Re: shimming engine bearings
 

Quote:

Originally Posted by norcalal (Post 1090028)
Model a engine had a rattle starting with just medium throttle in second gear. Center main was over .003 then, brought it to.015, spun fine, I took a shim out of one side of the rear main, now extremely hard to crank, but starter will turn it freely. PlastIguage shows .015. don't want to burn up a bearing, should I reassemble with too much clearance according to specs? Please advise!! thank you

If you want to be safe from a slightly tight bearing, pull it around the block a time or two.

Water and oil in, gas off, and spark plugs out.

High gear, and about 15 miles an hour, never more.

norcalal 05-22-2015 12:40 PM

Re: shimming engine bearings
 

Thank you for the replys. I am considering putting all the shims back in and starting over. The engine will turn over with 12v on a 6 volt starter, but I have to stand on the crank to get it to turn at all. I am worried that I will burn up a bearing even though the plastigague shows .0015. What do you think?

bbrocksr 05-22-2015 12:47 PM

Re: shimming engine bearings
 

Quote:

Originally Posted by norcalal (Post 1090212)
Thank you for the replys. I am considering putting all the shims back in and starting over. The engine will turn over with 12v on a 6 volt starter, but I have to stand on the crank to get it to turn at all. I am worried that I will burn up a bearing even though the plastigague shows .0015. What do you think?

I think you're right, Too tight.
Bill

BILL WILLIAMSON 05-22-2015 02:21 PM

Re: shimming engine bearings
 

:D
Quote:

Originally Posted by bbrocksr (Post 1090216)
I think you're right, Too tight.
Bill

An old drag racing trueism: "THE LOOSE ONES GO & THE TIGHT ONES BLOW":eek:
Another one: "IF IT DON'T GO, CHROME IT":D I don't know why I included this one, it just always made me LAFF!:D Like when a zippy, custom car was unloaded at a major car show & it wouldn't even start & had to be PUSHED into the building:(
Bill W.

ursus 05-22-2015 02:42 PM

Re: shimming engine bearings
 

The fact that your center main was out 0.003 tells me that there is enough wear that you should also measure the bearing races for ovality (out of roundness). The references to what is or isn't in spec for clearance are based a true round bearing and cannot be uniformly applied to a bearing surface that is no longer round.

The center main usually wears faster than the other two and I have learned to adjust it last. With sufficient wear, you may not be able to get the center main as tight as the others without creating a situation where the alignment is off a bit and your efforts result in accelerated wear and maybe a new knock.

PC/SR 05-22-2015 05:00 PM

Re: shimming engine bearings
 

Just wondering, did you oil the plastigage? If not, may have a false reading.

Chuck Sea/Tac 05-22-2015 06:16 PM

Re: shimming engine bearings
 

I understand there's a lot of controversy regarding whether to use oil or grease when checking plastic gauge. Does it actually say on the package whether or not to use oil?

CarlG 05-22-2015 06:20 PM

Re: shimming engine bearings
 

Quote:

Originally Posted by Chuck Sea/Tac (Post 1090389)
I understand there's a lot of controversy regarding whether to use oil or grease when checking plastic gauge. Does it actually say on the package whether or not to use oil?

I was always told that putting oil on would give you a false reading.

Having said that, I would think that putting oil down would certainly make cleaning the plastigage off after taking the measurement a lot easier.

My1930ModelA 05-22-2015 08:19 PM

Re: shimming engine bearings
 

Just last week I adjusted mine as well, on my 38,000 mile 30 Town Sedan. All three mains were shimmed with .006" on each side and I had a knock at fast idle, and measured .003"+ with Plastigage on all three. Removing a .003" shim from each side of all three caps resulted in .002" clearance, and that is as far as I felt comfortable going on this original engine. I'll find out this weekend if the knock has disappeared.

Greg Jones 05-22-2015 08:33 PM

Re: shimming engine bearings
 

Use a drop of oil with plastic age like Herm does. Makes a difference in getting an accurate measurement.

PC/SR 05-22-2015 10:10 PM

Re: shimming engine bearings
 

I know from personal experience that if you do not oil the plastigage that it will indent into the cap. The amount of the indent of course does not spread out and the result is that the Plastigage is not as wide as it would otherwise be, is narrower than if it spread to its full amount, and therefore will have a narrower spread, and hence give a reading of more than the actual clearance (the less spread, the more the clearance shown on the index. Is this making sense?)
I follow Herms method of spreading a film of oil the length of the plastigage on the journal, then put a small bit of oil on the cap and torque. The plastigage does not stick to the cap, and use a fingernail and laquer thinner to remove it from the journal.
1930 MOdel A: I do not understand how removing a .003 shim from an .003 clearance will result in .002 clearance. Did you oil the plastigage? Have you tried turning the engine with the crank or starter? Something there just does not sound right to me. Not trying to argue, just a bit worried that the knock will be gone but the clearances might be too tight and cause more probs. Let us know because feedback on all these things is beneficial to the next guy with the same problem.

Mike V. Florida 05-22-2015 10:31 PM

Re: shimming engine bearings
 

"Oval" and/or bent crank will give you what you are seeing as well. Use the plastic gauge turn it 1/4 turn and do it again, turn it 1/4 turn and go it again, turn it 1/4 turn and measure it a last time. All should be the same.

Mike V. Florida 05-22-2015 10:35 PM

Re: shimming engine bearings
 

Quote:

Originally Posted by PC/SR (Post 1090512)
1930 MOdel A: I do not understand how removing a .003 shim from an .003 clearance will result in .002 clearance.

What if you only removed it from one side?

PC/SR 05-22-2015 11:17 PM

Re: shimming engine bearings
 

Mike V: Good question. I just assumed .003 was removed from both sides.
If you remove .003 from one side and none from the other the cap will be angled to the low side, and somewhere across that circle of the journal there will be .002 clearance. There will be more clearance to the cap on the high side and less on the low side. The mathematicians and engineers here would know more about the effects and geometry than me.
Personally, I mic the shims and if off on either side by more than around .0005 or so I sand them down so the shims are pretty much equal thickness. I use some 400 grit and a flat surface. On the teardown that is going on now for the new Burlington crank I am using Timesaver to lap in the babbitt, and like the results.

My1930ModelA 05-23-2015 03:54 AM

Re: shimming engine bearings
 

PC/SR, on my initial Plastigage reading, I believe I had more than .003" clearance at all bearings, as the plastic did not 'spread' at all, using Plastigage with a .001-.003" range. I removed a .003" shim from both sides of the cap, and re-measured, arriving at .002" as measured by the Plastigage. This left a .003" shim present under both sides of each cap, and I felt this would provide for further adjustability in the future and the crank turns freely at the conclusion of my adjustment. We'll see how the engine sounds soon!

Mark

PC/SR 05-23-2015 11:45 AM

Re: shimming engine bearings
 

That makes sense. Thanks, and good luck.


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