The Stovebolt Site Forums The Shop Area Engine Shop Crankshaft shims for 235

Pulled crank out of my 235 casting #3769716. I've never done a 235 and I was trying to figure where the shims go. Should they be on all the mains or what.
Thanks
Larry

The shims are only used if the clearances are too tight. You would check the clearances using Plasti-Gauge, then add shims as necessary. Each journal should be done independantly of each other to determine whether a shim is required or not.

Thanks Bill, I figured that it must have been close because the main journals look pretty good and there weren't any shims on it anywhere. Rods are another story. I'm gonna have to have the crank turned.

3769716 is a 58-63 235 block casting code.

Were main bearing shims used in those years (were main bearing shims used from 1955-2nd onwards)?

The shop manuals do show the use of shim during overhaul through at least '60. They may not have done it at the factory, thoguh.

I have only rebuilt one post-1954 engine (a 1960 261, on which I used no shims - the engine is still running strong 38 years later), so my experience is quite limited.

The 1958 Chevrolet Truck Shop Manual states:

(page 8-22)
Main Bearings
.
.
Replacement (Engine Installed)

NOTE: If, for any reason, main bearing caps are replaced, shimming may be necessary.

(top of page 8-24)

NOTE: If new bearings cap is being installed and clearance is less than .001", install shims as required.

Maybe I am just not not finding the mention of using shims when installing main bearings, except in cases where the original bearing cap is not being used?

On page 8-28, the 1958 Shop Manual states that shims are not even used on rod bearings?

I do understand why/when someone might want to use shims to avoid having to get the crankshaft bearing-journals ground; but, I do not see this use of shims described in the 1958 Shop Manual (except for when a main bearing cap has been replaced).

I have been known to mis-read things, so I apologize in advance if I am missing something in the Shop Manual.

I know this is not the original engine from my truck because it had a 270 in it from factory. It's a 58 GMC but the only thing left GMC is an emblem on the hood and the dash the rest is chevy. I know a lot of you guys have lucked into some low milage trucks in the past but the fact that mine is showing 213,000 miles on the odometer and still not dead yet says something for the old girl. I wanted to put something different under the hood, even bought a small block BH. I even have access to an all aluminum Offenhouser V8 and a 4 cyclinder Cummins. Decided it was less work and would be much cooler to run the 235 that was in it.
Thanks for the input and I know I'm going to have to pick some of you minds for more later on.
Thanks
Larry

All factory assembled engine up to 1954 had shims on the mains. I have never seen shims on 1955 or later mains. No full pressure engines had shims on the rods.

My '55 had shims on the mains. Engine had never been apart before I got it.

Just a thought:
Bearings are round
Journals are round
Shims are not

an all aluminum Offenhouser V8

Details, please?

Offenhouser built the aluminum V8's for Buick. Friend of mine has one in his old racing jeep, mid 60's jeep wagon.

Offenhouser built the aluminum V8's for Buick. Friend of mine has one in his old racing jeep, mid 60's jeep wagon. I'm just not a fan of the aluminum engine unless it's in a VW.

I believe that when they switched to main bearings without shims, the dowels were gone too. I understand that the dowels were there to stop the bearings from spinning but without the shims there was the crush affect to take care of that.

Dave, shouldn’t you include the ’54 in with those full pressure engines rather than ’55 and up? May be I just miss understood what you were saying?

Most manuals or papers will describe main bearing fit as “selective fit” which means that shimming may be needed. The ideal starting point is a perfectly line bored block and a perfectly straight crankshaft. This is an excellent paper from Clevite to establish a starting point. http://www.mahleclevite.com/techbulletins/CL77-1-205R.pdf But nothing is perfect so that’s where the selective fit comes into play. A couple of tenths here and a couple of tenths there and before you know it the crank is binding up so you add a shim till it turns free.

Panic, yes the question often comes up about shimming which results in an oval bearing bore rather than a perfectly round bearing bore. You will notice that main bearings will show most of the wear on the bottom (cap) then the top bearing shells because that is what takes most of the load. The rod bearings will show more wear in the top shell then the bottom (cap). Because the majority of the load is applied at these points their clearance is most critical. These areas need the most support from the lubrication to avoid metal-to-metal contact. Since the loading is less on the side or at the parting line you can live with less clearance therefore less thickness in the oil film.
Denny Graham
Sandwich, IL

RE:Offenhauser

I don't think Offenhauser "built" the aluminum BOP engines for GM, but they certainly could have been involved in the block casting process as GM had major problems with that engine. Today I think Rover owns the rights and uses that engine. However, because of it's light weight and potential power,there are aftermarket builders of these motors that are quite successful, especially in the modified sports car racing groups. Not related to the original subject, but thought I'd throw it in.

So did mine.

Denny

1954 (and in at least 1955-2nd series) engines used shims for adjusting main bearing clearance. However, shims were not used to adjust rod bearing clearance on 1954 (and 1955-1st) engines.

The 1956 Truck Shop Manual states (sect. 6.2,):
MAIN BEARINGS
.
.
The adjustment of shims formerly used between the bearing cap and cylinder block have [sic] also been discontinued.
.
.

The statements above (and in my earlier post) are from Chevrolet Truck Shop Manuals. I have no doubt that many shops used shims even when a Shop Manual stated they were not necessary. Many shops also filed bearing caps, even though the Ship Manuals clearly warn against doing this.

The engine in question at the start of this post is a 1958 or later engine.

There would be no reason to shim the bearing caps on a properly line-bored engine. The shims were a production gimmick to correct for less-than-properly machined bearing bores or slightly off-sized crankshaft journals. I like to bolt up the bearing shells in a block without the crankshaft installed, and measure the actual ID of the bearing, then subtract the shaft diameter from that reading to get an oil clearance figure. If a little more clearance is needed, I add a shim, but only on the engines with the dowel pins in the main bearing bores, to prevent the bearing from spinning. Newer-design blocks without the dowels definitely don't need to be shimmed, as it interferes with the proper crush fit of the bearing in the block. Those raised tabs on the bearings with the corresponding slots cut into the block and cap are called "anti-rotate" tabs, but with the right crush fit, they're not really necessary to prevent bearing movement.
Jerry

Buick built the Buick aluminum engine for Buick.

The shims were a production gimmick to correct for less-than-properly machined bearing bores or slightly off-sized crankshaft journals.

Definitely - use an expert to adjust it on the line rather than re-machine the parts, but of course the result is an oval bearing bore.
Curiously enough, making the bearing's major axis in the opposite horizontal plane before honing to size with a shim is a useful speed mod for very high RPM.
There is evidence suggesting that the connecting rod’s big end bore (which determines the shape and final size of the bearing shell) is not round at high speed & load, but oval, and stretched vertically by perhaps .001”. Bill Jenkins, one of the most successful Chevrolet V8 tuners of all time, used this information to his advantage when he prepared steel connecting rods for racing. He inserted .002” shims at the “parting line” between the connecting rod body and cap before re-attaching them for resizing. The bore ID was then honed .002” oversize. When the shims are removed the final bore size is nominal in the vertical axis, but .002” wider at the parting line, and will “pull” into the correct shape at high speed.

I figured that out too. I was going by hear say and the name on the intake. Still glad I didn't go for putting it in my bolt.
Larry

Now I know why they don't send donkeys to college.
Jerry