Is there a side gap spec for connection rods?

bobbycoke

bobbycoke

Senior Member
Have an engine apart that was machined and rebabbtted ......not buy me in trying to reassemble the rod caps are too tight [side to side] on the crank journal. The fit on the crank plastic gauges ok in tolerances but when torqued down lock the crank up.......Rod Caps all measure 10-15 thousandths larger than the crank journal width so an interference fit......Is there a spec for this rod cap side dimension or is it just enough to not bind??????Thanks Bobbycoke
 
Since you have an interference fit the sides have to be trimmed to a clearance of about .004 - .007. Frequently much larger side clearance goes unnoticed with no issues. Likely best to have the babbitt shop do it. After rereading, how did you use plastigage if the rods & caps are to wide? You may have a radius issue.
 
Are you sure about the side clearance readings in rods and between the cheeks on the crank ? That has to be done with a micrometer and snap gauge.
 
No radius issue, the caps were put on and torqued with plastic gauge the radius was fine , they were hammered on [soft blow hammer to start then nuts to compress] not by me this is a youth restoration project by our club [Adrondak A's ny] our 5th youth session please feel free to check website......they then would bind the crank we checked with a vernieer caliper and a micrometer so we know they need machining just in all the books and standard could not find this spec......thanks for quick reply,,...... we will recheck the radiuses after the machining ......the youth get impatient but realizes that the only way to rebuild an engine is the right way ...if you do not have the time to do it right, how are you going to find the time to do it again?>>>>....... bobbycoke
 
Bearing to crankpin clearance is done with Plastiguage or micrometers, .010 - .015 clearance is way to much clearance for a rod bearing and this is not side clearance. I guess side clearance is not such a issue now because forcing the bearings on would have sheared the sides of the babbitt on the rod and left a zero clearance or a broken babbitt side. That would cut into the radius on the poured bearing. I was a auto shop teacher for 10 years, I feel your sorrow. Tough to describe and likewise on this end, tough to visualize 3000 miles away.
 
The STOCK width on a Model A rod is 1.625, -.007 thousandths, or a total of 1.619 wide.Unless somebody opened them up some.

One point here, everything can seam out of specs., when the Rod is out of Alignment.

And you should go no further until they are checked for twist, bend, and off set.
, because if you haven't, they can be bad enough, they won't fit just because of that. I will guarantee the people that rebabbitted them didn't.

When that is done right, then you can check the fit.

Shaft clearance should be .001-60 at the lowest, and .002 thousandths at the highest.

With Spec's, they should have , .005 to.007 side play.

All hole sizes should be checked with a inside Mic, or sometimes called a Post Mic. I will show a picture, if I can find one.

With a post Mic, you can measure right to the 1/10 of a thousandths every time, after time.

Set the Post Mic, and then read the out side with a normal Mic.

The worst, uncertain inside Mics are snap Gauges, they will not read 1/10's, as they are all over the place, in different readings every time.

Anybody, when you put on rod caps, make sure the cap has its small tapered hole facing the correct direction. The dipper points to the taper hole, closest to the part line, or away from the tapered hole Further from the part line.

Herm.
 

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From Ford Drawings: ROD: BABBITT WIDTH-TO THRUST FACES 1.614-1.616
ROD JOURNAL WIDTH 1.620-1.626
THEREFORE, ROD END PLAY .004--.012
I have gently filed these into specs. Not the best way to do it, but.....
However, you got to consider crank end play. If the crank end play moves more than the rod side clearance, you have to check the clearance of the rod small end on the pin to the inside of the piston.(I think .040 is the minimun small end clearance.) Not enough play there and things will start to bind up. Check the rod side play clearance with the crank at full movement in both directions to the crank thrust faces.
 
Ok, as per some e-Mail Questions.

There are two tapered holes, one on each side of the rod part, right above the bolt on each side.

The dipper hole, in the cap, faces the side of the rod that has the tapered hole closest to the part line. When the rod is installed on the crank, the dipper is facing the Camshaft.

These tapered holes were used for holding the rod to machine. They are also handy to see which way the cap goes on the rod, if they are not marked, or if they were installed wrong when you bought them.

Thanks,

Herm.
 
Herm, does it really make a difference which way the tapered holes are, as long as the cap is on the rod the correct way so the oil lines match? I have two done the way you say, and two the opposite. I don’t see how this would affect anything?
 
There is a Babbitt shop in Mo. that throws all Model A Rods in one shopping cart, and all Model A caps in another shopping cart. So when the rods are assembled again, the caps and rods do not match, or have any markings that match.

Also the caps are a miss match for rod width for matching the sides, bolt spread, and then rod assembly markings.

If then, the rods are put together the way they should be, before being machined, all you have to do is a quick glance at the dipper direction, and the hole, you need no other markings, unless for which hole they come out of, unless you are doing one at a time.

We rebuild every kind of bearing , rod, insert, cam, ect. Some need to go by markings, some by direction, and some bearings will not even let them selves mismatch in assembly.

As far as having two rods with the cap used wrong, If the mechanic knew a brand new set was like that, he could send them back. If I had seen that a cap was the wrong direction in an engine, I would assume that the Guy that had last adjusted it, screwed up.

If you hit the Rod square in the bore when machining, the cap will go on both 1/2 turns, of the cap. But even a 1/2 thousandths will lock it up.


Herm.
 

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I agree with Herm. With a engine good enough does not work. It needs to be right. Things were done at the factory for a reason. The more you get away from right the worse the engine will be and the more noise and vibration there will be.
 
not to interrupt this tech . if rods are to thick (side to side) I would use fine sandpaper on a flat surface & sand both sides equally to get the correct clearances , JMHO
 
Hi Gents. Ive found this thread very helpful. Thank you! I have a question if anyone has the time. My rebuilt engine has developed a noise low down that I am not too pleased about. Fearing the worst, I removed my sump today. So far, without removing end caps I have discovered that my rear connecting rod has a 'tight' 10 thou' end play gap. From your thread, I figure this is at the outer limit. My engine is an OHV conversion via Neal Jern creating as much as 70 HP (according to the late Neal, RIP Sir) My question would be: Is this tolerance too great and any ideas what I can do about it? I'm from NZ, so my go to solution would lead me first to my spare connecting rod bin in search of a better fit. Any advice would be greatly appreciated.
 
Buick 300, The side to side clearance on the rod will not cause your noise. If the rod is aligned and not bent there is nothing forcing it to move back an forth. However, if the rod is twisted or the ends are not in alignment, the rod will move back and forth, causing a noise. Have a reputable automotive machine shop check and correct the alignment on all the rods.
 
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