I'm assembling a '55 261 that's 10 under on the mains. The chevy shop manual says to use shims on both sides of the main caps and remove shims to get the right bearing clearance. I've also been told that the block and caps were factory line bored and shims should not be used. If I tighten the caps with no shims, the crank is locked in place. The counter-bores mic slightly oblong (wider than tall) shims may correct it I haven't tried any. If the caps are torqued without the bearings, the parting lines can be felt, but these caps and this block were a running engine to start with. I've got conflicting advice on shim vrs line-bore. Have you had this sitution and what did you do. Shims are cheap next to the line-bore, but I want to be right. Thanks. joe

Did the engine have shims originally! Many of those engine use shims to "dial in" your clearances. I've even seen service bulletins say that if your oil pressure falls off below a certain point, that an "adjustment" to the shims by your mechanic can bring back lost oil pressure. So I think that shims are an acceptable method for "tweaking" your clearances. Its just a little more time consuming. Most times, before a crank is turned, you can torque the bearings in the block and caps without shims, and have the crank turned to fit it,eliminating the shims altogether.

First or second series '55 engine?

First series is like the '54 and uses shims...

It's out of a second series truck. But, the glock and caps are for doweled bearings which I think were used only in the first series. That's part of the questions. Should have added it to the first post. The counterbores are undersized in the verticle about .005, and they are about right side to side. The book for first series says to start with .006 shims and work from there. That would say it's most likely a first series motor. I"m pretty sure it's the original motor for the truck, but by that age, who knows. I'm taking it to a shop run by an old guy (even older than me and that's a lot) who has worked on these engines for his career. I'll see what he has to say. Just hate to spend $ on line boring if a set of shims is all that's needed.

The type of water pump would tell if it is a first or second series block. The second series has one large hole for the impeller to go into, the first has 2 smaller holes and the pump sits up higher than the top of the block.

I'll try digging around for some more info. Perhaps the second '55's had shims also, and they went shimless in '56...

Well, looking at my master parts catalog, its a little convoluted, but I think they used both types on the second series engines.

Not that it matters, it is what it is. Get the older type bearings and shim the caps.

As gearhead says, the main bearing installation is a little convoluted. The 1955 second series service manual does not mention the use of shims. As expected, the 1954 shop manual does describe the need for main bearing cap shims. The master parts catalog shows a part number for shims for all 1937-57 six cylinder engines, as I read it.

For further confusion, I have two early (1954 or first series 1955) 261s that have the tang-type of main bearings with no shims. To the best of my knowledge, both of these engines were purchased as replacement engines; Chevrolet must have upgraded the engines when the replacements were assembled.

In any event, I agree with gearhead's advice.

Does everyone use the terrific website that Keith Hardy has assembled?

Should have added, that the water pump shows it is a second series or later engine. Also, the 1955 second series service manual includes a discussion about shims and adjusting clearances with them in a different section than the section referred to above.

Here's the latest. The shop that bored the block was going to assemble the crank, rods & pistons and cam --- doesn't want to do it using shims. The "old guy" I hauled the block to for line boring doesn't think it's necessary and suggests using shims, however he'll bore it if I want.

I drove a 180 miles each way today and still don't have a clear feeling for what to do. I'm leaning toward making the trip again to bring the block home, gathering all the other parts back from the first shop and assembling it all myself.

Now it's time for a whiskey and to think it all over again.

one of the things we did was call Patrick Dykes and found out all of the differences between the years. Here is the bottom line: Often times the production years werent always 100% the way they were supposed to be. So you very well could have had shims in the second series, even though they "should not" have been there.

Patrick also advised me to pitch the shims and line bore the blocks that had shims.

If you were building a stocker (ie resto) and determined it was supoposed to be shimed (from the factory) then I figure that's fine.

But if you are building one that's souped up, I would line bore it (as required, weather the factory shimmed it or not).

But that's me.

Thanks to everyone for the info. This is a great resource for those of us that have forgot more than we remember.

I got the same story from Patrick. I've run into it before with a GMC in the late sixties. I had a factory installed engine that was "never used" in pickups. Factories do all sorts of things that are contrary to what they say.

This won't be a stock engine, but it won't be a hot engine either. More your standard 40 over, moderate cam, dual carb 261.

The block will be line-bored, that way if I'm lucky enough to wear this one out, It'll be ready for next time.

Thanks again. joe

Hey Tom, If the block had shims, and it is line bored, what bearings do you use? John

they are either doweled or tanged, depending on year of manufacture. Call Patrick Dykes - he will step you through which bearing you need

can someone explain how you align bore a block with all different sized bearings?

You have to use a boring bar to machine one housing bore at a time to their respective sizes. Since each main housing bore has a different size,you'll need to switch between 4 different sized OD rings to support the bar in each main saddle as you selectively begin to bore them. You might be confusing this with align honing the mains, which takes a long hone and hones the housing bores all at once, and to do that all the housing bores need to be the same size to accomplish this, and cant be performed on engines such as these.

Interesting questions about how to do it. Same ones I had. Turns out lots of newer shops with younger people don't know how to bore four different size journals. In Montana, I could only find three shops who would bore (as opposed to hone) the mains. One of them only works on large diesel engines. The shop I used was 3 hours away and is the son of the original owner. Guess who came to the shop to do the measurements and set it up - - OLD GUYS RULE. He pointed out that the most significant thing to pay attention to was the center-center measurement between the cam and crank journals due to the gear drive for the cam. By the way, Patrick helped me find this guy

The internals go into the block Monday.

Patrick is a great guy! Glad to see you are making progress with your engine....

Time for an update of your progress, please?

Nothing out of the ordinary to report on. The boring went good. Crank, rods, pistons and cam all assembled just as they should have. I'm working on putting the clutch and tranny on the block so I can set it in the car and locate the places for motor mounts. In the mean time, I asked some questions about rocker assemblies for 235 heads vrs 261 heads. (there's a message thread somewhere). Learning as I go, it all seemed less complicated in the 60's. Close enough was close enough for me then.