VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 Adjusting tightness of '30 connecting rod caps

If you read my recent post on dropping the oil pan for my '30 engine, I cleaned it thoroughly of a guart of sludge, and also semi-cleaned out the troughs above front/rear bearings. All 3 main bearings seem tight, no detectible sign of movement, so I plan for now to leave those as is.

I'm now looking for further advice on adjusting the connecting rod caps (manual is pretty thin on advice). Per previous Chat advice, 5 of the 6 rods could be moved laterally by hand, suggesting a bit too loose (#4 rod seemed tighter than the rest).

One by one I removed the caps (they look in perfect condition, no signs of wear or scoring etc) and each had 2 brass shims per side, a super-thin shim and one a touch thicker. For some of the caps I could remove both sets of shims and tighten up the bolts and still turn crank, albeit somewhat stiffer. A couple I left the thinnest shim in place. That's all I did for first round.

So my questions are many:
1. I plan today to fine tune the tightness of all 6 by ading/subtracting or leaving as is, any further thoughts on just how to get tightness bang on? (I have the 8 oz hammer tap advice, thanks Chev).
2. Is there a recommended torque for cap bolts? I realize they are cotter pinned, suggesting torquing is not essential. The bolts were quite tight when I removed them, took some effort on a 12" rachet to get them loose.
3. Once all are tightened and pinned, my plan is to replace oil pan using the old gasket temporarily, add new oil and start up the car to see what things sound like. If all is well, take it for a short spin to work things in slowly.
4. If everything is good I will remove pan for painting. I see rust in about 1/3rd of bottom of pan, not too serious, but I would like to do something with it to prevent further corrosion etc. Any product recommended (Rust convertor, rust paint, etc).
5. Once pan is restored, I have a NOS gasket and I see advice elsewhere on installing pan and new gasket.
6. Any further cautions are also welcome.
Thread a bit long, but hope to hear back. Would have posted last night (Tuesday), but ChatII site was down I guess, could not get on between 10pm and 11pm local time.

Last night I believe we switched to a new server and so far everything seems fine. Regarding your rod fitting I would follow Genes advise carefully removing shims until the rod does not move when tapped then add a thin shim back in. I don't believe rotating the crank is a good way to check the rods. In an old book I have it actually explains the process of tightening the bearings to a point of drag and then burnishing them in by turning the crank with an outside source of power, sometimes even another operating engine. If you have a rod that all shims are removed and its still loose you will have to resize the rod, probably by filing the cap. If any of this information is incorrect I'm sure Gene or others will correct it.

Gunsmoke,
Your plan is reasonable to me.

Well, guess what? It appears to me the rod dippers were installed backwards! The engine turns clockwise (if I stand facing front of car) and open end of dippers should face passenger side if I read things correctly. All of the dipper open ends were facing the driver's side. I see the manual warns against this mistake. I am guessing this is at least a partial source of the noise in bottom of engine caused by insufficient lubrication of the rods. While that may be the bad news, the good news is it appears these caps/rods were newly babbitted not long before car was laid up for 45 years (partly accounts for sludge in pan as oil has thickened over 50 years). I took one cap to local engine specialist and he was surprised how good they look if dippers were in backward.

He suggested I use plastigauge as a double check for fitting and suggested 2-3 thousands as ideal? He also suggested to torque cap bolts to 35 ft/lb, and then move a touch tighter to closest hole for cotter pin(about 40 ft/lb when finished?).

So that is my plan for now. I notice the oil hole in the cap is offset slightly off center towards one bolt. Since I have to put the caps on exactly as they came off, I assume reversing only the dippers will not cause any issue related to hole location. Was the oiling hole supposed to be on one side (pass or driv), or does it really matter?.

THE OPEN MOUTH OF THE DIPPER FACES TO THE PASSENGER SIDE OF THE ENGINE..If your dippers were backwards bearings would have burnt out by now. The oil hole in the cap will be towards the drivers side.
Rod caps can not be installed either way as when the rod bearing was babbited and bored it was done with the cap on the correct way.
Do not listen to your "modern" mechanic. .002" to .003" clearance will be way too loose and rods will be knocking. A properly adjusted bearing on that engine will have .001" to .0015 and they only accurate way to do it is to follow the Chevrolet shop manual.

I agree. Your engine "specialist" doesn't have a clue as to what he is talking about.

Ok gents, calm down, he suggested 2 thousands should be close enough, but said he had not worked on an engine like mine for a number of years. So you guys suggest aiming for about 1-2 thousands, sounds like a good target. Problem is figuring out how to know when I am close when I have no such experience.

4 methods have been suggested, 1. use plastigauge, 2. follow ChevGene's (or Bill B's) light hammer test, (which is likely very reliable if you are experienced) 3. follow manual approach (which is not very precise), or 4. another local engine fitter who suggested removing a shim at a time from both sides of one rod and torque up, etc until engine will not crank by hand and then adding 1 or 2 thousands of shims until hand cranking can occur. And repeating one by one for rest of the rods.

I am doing this wonderful job from under car sitting on 4 axle stands so wheels are about 8" off floor. Can have a snooze on the creeper!

So I will aim for .0015. Does 35ft/lb torque sound right for cap bolts?

Finally, indeed the dippers were in backwards, one still is as I have only adjusted 5 rods so far. Hard to assess if any serious damage was done (no obvious scoring or wear on caps), I have put about 200 low speed miles on car since I got engine running last year after sitting for 45 years+. It could be that at that time 45+ years ago the engine was refitted (with dippers put on wrong) and never driven, and thus the backward caps did not have enough drive time to cause measureable damage. The existence of 1/4" of black sludge in pan suggests engine had likely sat for a long time unused. I plan to check upper half on rods tomorrow before finally buttoning things down.

Shoulda gone to mechanic school I guess. If any of you guys want to drop by to further assist, I will wait for you with a cold beer.

I bet you are glad that you decided to drop the pan and inspect sooner than later!!

Nope.....not two thousandths....that is too much. The specs. that I have show that the ideal clearance would be 1/2 of a thousandth to one thousandth. One and a half thousandths would be max. However, I had a rod knock with one and a half thousandths clearance.

The torque listing that I have calls for the connecting rod cap torque to be 40 to 50 pounds.

If you follow Gene & Bills instructions you will soon be the expert. If you remove a shim and there is no movement with the hammer tap then you are at zero clearance. Add the .001 shim and retest and you should be good to go. If you take out all the shims and still have movement then resizing comes in to the picture as until you get to zero there is no way to know the clearance. I'm sure Gene or others have the ability to tell by the hammer tap pressure, but I would have to use the above method since I haven't done so for 50 years or so.

I used Gene's method and found it works super. After finishing the work, I looked through some of my Dad's old manuals and he had written basically the same procedure in the margins of a 1933 Motor Manual that I have.

This advice is all of great help. My initial concern was getting them too tight which is also not good. I'll aim for about 1 thou and see what I can do. From what I can assess todate, they probably had 4-5 thou per rod and that caused 2 things, a lot of noise under revving, but possibly saved the caps from overheating from loss of oil/friction due to dippers being backward. And yes I am glad I took pan off, for a while thought I may wait 'til next year! Right now I have them set at about 2-2.5 thou, and have about 20 thou of shim stock I have removed from 5 rods or about 2 thou per rod removed. I'll be a quasi-expert for the next engine.

Hello All,
May I ask, well, an I don't know question? Is, in all the shim checking and torquing of the piston rod caps, is this done dry, without oil? Would the clearance check be invalid with oil involved? If checking is supposed to be done dry, would you want to pre oil everything for the final assembly? I have no experience with working with shims, but wish to learn. Thanks

I have done it either way and never noticed a difference.

Today I took off #4 rod cap, the only rod not able to be moved laterally by hand. Leaving shims in place, I used some plastigauge and this rod has a clearance between 2.5 and 3 thou. The shims on this cap are about 3 thou total.

So it appears while this tightest rod was set at 2.5 to 3 thou on the caps, the other 5 rods were between 3.5 to 4 thou. So tomorrow I will set them between 1-1.5 thou, and see how things turn out. This along with getting the dippers facing properly will hopefully get the engine purring again. Fingers crossed.

Get the rods closer to 1 than 1.5 or you might have a knock.

That's my aim JYD. Do you use plastigauge or other tried and true methods? I expect experienced engine builders (like ChevGene)know this process so well they can do it blindfolded and just by feel, but us 68 year old rookies need a lot of practice. If I get it down from circa 3-4 thou to 1-1.5, hopefully that will do the trick for a car intended for mostly sunday cruises. I was surprised tolerances are that close for a low/no pressure oiling system.

On the old poured babbitt engines the close tolerances are critical. With a brand new engine I had a rod knocking at 1.5. All of the rods in my engine are set up between a half a thousandth and one thousandth.

I'm curious about the tolerances usually adhered to in milling the journals and boring the babbitted rods. Any runout expected in milled journals or are they expected to be perfectly uniform all around? Same for conn rod, perfect uniform bore? Third question is on how they are bored. Does the machinist put a certain # of shims between the cap and rod before boring to say 1/2 thou over size? Or are they bored without shims? My assumption is they are bored with 2-3 thou shims (laminated shims) in place so as the conn rods wear, shims can be removed to restore the 1/2 thou tolerance? Is this correct?

My 32 engine was rebabbited both rods and mains. The shop that did it eliminated the shims and set the tolerances at half a thousandth. His reasoning for no shims was that if the bearing surface wore removing shims would only change the top and bottom setting-the journal would become oval shaped and the only way to correct the situation is line boring the mains and resizing the rods.

What they said is logical but doesn't work in real life.
Also putting bearings together with no shims means a major repair when the bearings loosen-up.
When new Chevrolet placed two copper shims (.002") and two silver shims (.001" each )on each side. It was common to adjust the rod bearings every 30,000 to 40,000 mmiles in the early 1930's. Part was due to wear and part due to the soft, thick babbitt compressing. The thick babbitt was used so as to allow it to imbed microscopic pieces of dirt with out damaging the crank journal. This was the norm for all engines back then.

I had some concerns regarding the items you specified. Their response was that the babbit of today is much better than years ago. Also if adjustment was needed in 30-40 thousand miles at the rate I planned on using the car (around 1000 mi. per year) I would be 90-100 years old. If I found it necessary at that time then the caps could be ground a few thousandths and the rods resized to go another 30-40 years.

Babbitt is better today? Could be "harder" than what Chevrolet used but that doesn't make it better. The other problem is if the crank journals were turned to an undersize the babbitt is made thicker still and more apt to compress. Years ago we had a lot of problems where a crankshaft was turned to an under size and the thick babbitt pounded out in 5 or 10,000 miles. Today if a crank is under size the bearing insert in made thicker and the babbitt the same thickness as original. One advantage of inserts.
Then with limited driving and todays better oils they all should give adquate service.

I also assume the polished steel of the journals is much harder relatively speaking than the poured babbitt (which is in essence a sacrificail material). Therefore friction wear if any will consume the babbitt first. Impact distortion is most likely more critical on upper center half of babbitt (in the conn rod and not the cap) due to force of power stroke on babbitt. That is the only time the babbitt gets a load other than normal friction from rolling around the journal. the power stroke is likely what causes most of the eventual wear/material compression, and why shimming need only be up/down and not side to side.

Very true. If there is babbitt failure due to pounding it is always on the top half (the rod and not the cap) and on the main bearings is on the bottom (cap) and not the top.
Over the years they made the babbitt thinner and thinner. From 1948 and up it was the thinnest and they could go 100,000 miles without rod adjustments with good maintainence (oil changes).