Piston Rod Clearance question

[Fred K-OR]

This is the old guy that does not know what he is doing and thus needs some more help.

I have my "new" engine (that I acquired lately) torn down to the point where I can check the piston rod clearance. I took the rod caps off, used the plastigage thing and if I did it correctly, got a reading of between .002 and 003 readings on all rods. All the rods had one shim in them on both sides.

Just to check if I did it correctly, I cut a strip of the plastigage, put it between the piston cap and the crank, tightened it down to about 35 lbs and then took it off. Pealed the plastigage apart and measured the splotch on the paper. Like I mentioned above, all 4 rods have a reading of between .002 and .003.

I know this is only one factor to check, but looking at the other things on this engine, it "seems" like they are in pretty good shape. The compression was checked and it is good, it had no oil leaks, timing gear looks good, etc.

So the question is: Will this engine based on these rod readings, and a non-professional mechanic's ideas of a good engine, give me a reasonable length of service without a complete rebuild? Can I get a few ideas and feelings and if you like you can take some pot shots at me for being so unprofessional.

Thanks for any help you would like to offer.

[Gary WA]

Suppose to remove the plastigage material from the paper and apply. not with the paper attached, if I read this correctly? You left it on the paper??

[Fred K-OR]

Originally Posted by Gary WA

Suppose to remove the plastigage material from the paper and apply. not with the paper attached, if I read this correctly? You left it on the paper??

Yes I left it in between the paper. Like I memtioned, I don't know what I am doing and do appreciate the help.

So if I understand it, you take it out of the paper and then do you put it across the rod or wind it sort of around the crank or inside the cap?

Thanks

[Ray in La Mesa]

I don't want to step on Gary's toes, but the way I do it is to cut about 1/2" of the green Plasicgauge (.001"-.o03") and lay it longitudinally along the bearing and torque it. Remove cap and measure width. I aim for 1 1/2 thousands.

[Aerocraft]

Fred: It doesn't take much, I use roughly a 3/8" long strip, lay it across the journal and install and tighten the cap. If the clearance is greater than .002", remove a shim from only one side and try it again. Shoot for over .0015" and under .002" and that journal will be fine. Continue with all seven journals and your ready to go. Gar Williams

[Tom Wesenberg]

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Just to be perfectly clear, lay about 1/2" of the green string ONLY so it is from side to side on the bearing. In other words it lays perfectly flat, and not curving around the bearing surface.

[Fred K-OR]

Thanks guys for the info. Wow did I screw up. While I was waiting, I googled plastigage and it told me how to use it. Looks like I will have to go out to the shop tomorrow and start over. Hope I have enough of plastiguage left to do my checking-I sure wasted enough of it to do two engines.

Once I hopefully do it correctly, what reading is OK to continue to use the rods? Is up to .003 still OK? All of the rods have one shim in each side. So guess I could take the shims out and check it again.

Thanks again.

[James Rogers]

.003 won't work, it will knock and go bad in short order. You need to be at or less than .002 to be good. .0015 would be the best.

[Kohnke Rebabbitting]

Clearance for any shaft is .001 thousandths per inch of shaft, minimum, and or plus 1/2 thousandths Maximum! The most important thing is to use a couple drops of oil, on the shaft, and cap before you tighten the cap, or the Plastigage will not spread out, and will give you a false reading.

Lastly, always read the crank, never the bearing, on a Babbitt bearing. After a 1000 miles, or so, your rods will end up at 2-1/2, to .003 thousandths again, as the crank swells, but you don't want to start there. Herm.

[Fred K-OR]

Thanks James and Herm. That is what I need to know. (Once I use the plastiguage correctly)

[40 Deluxe]

Also, be careful taking the cap back off. Just pull it off the rod bolts. If you tap on the bolts to get the cap off, you are forcing the Plastigage againt the crankshaft which flattens it further. This will give you a false reading as the flattened Plastigage may read .002 when you actually hammered it down form .003 or .004".

[hardtimes]

Originally Posted by Fred K-OR

Thanks James and Herm. That is what I need to know. (Once I use the plastiguage correctly)

Fred,
The (possibly)good news is that you put paper is there ..with the plastigage...and still only got 2/3 thou ! When you do IT right without the paper in there...I think that you are going to have a smile on your face,eh JMO !

[PC/SR]

Fred, when you say you have "one shim" I wonder if you really have one shim pack, which is multiple shims stuck together that can be separated. The shim pack can look like one thick shim. Individual shims are .002-.003 thick, the shim pack can be up to .032 thick, or less if someone has peeled some off.

[George Miller]

Don't forget to check the mains. The center main and the rear main might need to be adjusted.

[James Rogers]

Originally Posted by hardtimes

Fred,
The (possibly)good news is that you put paper is there ..with the plastigage...and still only got 2/3 thou ! When you do IT right without the paper in there...I think that you are going to have a smile on your face,eh JMO !

I don't agree. The paper will take up room and give a flase reading. When the paper is removed with a reading of .002, the reading is probably going to be .004 or more. Remember, the paper is .oo2 thick and it was doubled. The plastigauge is only wax and paper won't compress much.

[Fred K-OR]

Well I went back today and did the plastiguage "correctly".

The following readings were what I recorded doing plastiguage the way it should be:

Rod # 1 .001-0015 Closer to .0015 with shim in of .032
Rod # 2 .0015-002 Closer to .0015 with shim in of .032
Rod # 3 .0015-002 Closer to .002 with shim in of .032
Rod # 4 .0015-002 Closer to .0015 with shim in of .030

Rear Main .0015-002 Closer to .002 with shim in of .021

Have not checked middle and front mains.

BTW from this "untrained eye" none of the bearings I took off seemed to have not much wear. Rear main was not cracked like I was warned to look for.

So that is the diagnosis so far of this guy that knows little about this situation. But my guess is that I am looking pretty good???????

Please give your guess. I know it would be hard to do this without doing what I did but your feelings would be appreciated.

Thanks again.

[Kohnke Rebabbitting]

No. 3 Rod Fred, you can take out a .002 out of your adjustment side, and put back in a .001, that will close it up by 1/2 thousandths, and you can always put it back if you don't like it. Herm.

[hardtimes]

Fred,
Wow, good on you and looks like I made a 'lucky guess', eh!

[Fred K-OR]

Originally Posted by hardtimes

Fred,
Wow, good on you and looks like I made a 'lucky guess', eh!

Yes you did and thanks for the encouragement.

[hardtimes]

Originally Posted by Fred K-OR

Yes you did and thanks for the encouragement.

Fred,
YOU give a lot of us 'shadetree' mechanic guys...a good name and encouragement !
Personally, I took away a lot from your story/efforts. The posting of Herms' pics of proper plastigage process ...to walk a guy thru the process ...are priceles(as in a pic is worth a thousand words.)
Keepem coming, as we all learn ,about Model A repairs, from such.
No dumb questions,eh