['ol Pinion head]

Very nice to have a 45+ year Pontiac engine builder that had/has the foresight to save main caps off even common 3' main Pontiac V8 blocks.
Ones from engines that prev threw a rod through the cyl wall or were freeze cracked. In the last 2 years I pulled the rear cap off a really rusty '73 455 core short block that
id had in the back of the shop for too long. The only thing i was hoping for to come out when i bought that short block, was the timing cover a decent 455 crank.
Was happy to give that rear cap to a friend needing it.

[John Milner]

Thanks again for your help with that rear cap, Roger. You helped save my 428. Will be more than happy to return the favor someday if I ever can.

[tempest1964]

Stories like this is why I like this forum.

[sdbob]

Yes.

[NeighborsComplaint]

Originally Posted by sdbob

I think we ve all done things when we were learning we look back and say how did I do that. I myself wouldn't be condescending.

We're not talking obscure parts interchangeability issues, this is fundamental machine parts assembly and plain common sense.

We all have Smartphones these days and accepted methods of disassembly and reassembly (even if generalized) are right in the palm of your hand. Google "How do you seat a main bearing cap on a Pontiac V8?" and here is what you get:

"Seat the main bearing caps by carefully aligning, lightly tapping, and snugging them before torqueing in sequence, ensuring proper clearance and thrust bearing endplay."

Before Smartphones, you bought a Chilton's Manual to consult.

[Dragncar]

Its cast iron it can always crack if you try and force it too much. Don't rush it, seat it first.

[ThreeNuts]

Originally Posted by NeighborsComplaint

First off, what you did was just dumb. You never ever force precisely machined parts.

Were I in your shoes. I’d try the new cap with the old bearing on the motor. Clean the bearing and crank journal with brake clean and apply a light coat of machinist’s blue on the lower bearing half. ...

Oh don't I know it.

Knowing myself this is exactly the thing I wanted to try but lacked the proper steps to do it, so I will acquire machinist's blue and give it a go. I am highly skeptical but absolutely worth a try with nothing to lose.

My other concern is that this is the seal bearing, whether or not the seal groove will line up properly. Presumption(!) is that should I get a good clean install then chances are the groove is also aligned.

I did locate a machine shop in Birdsboro, PA that seems to have the right combination of competence, equipment, and time to do this job should I not be comfortable with the fit of this new bearing cap.

Thank you again, everyone, for your input, comments, time and expertise. It is deeply appreciated and very helpful (and supportive!).

[tempest1964]

Good luck, and please keep us posted. Enquiring minds want to know!

[NeighborsComplaint]

Originally Posted by Cammer-6

without a lot of fuss you could try using plastigage to see if its centered

damn .... you got me there.

[NeighborsComplaint]

I thought about the plastigage a while and don't know how this would measure runout or eccentricity unless you rotated the crank and checked it every 90 degrees of rotation.

[sdbob]

My mother worked for a very good gynecologist. My father and I worked at their home many times remodeling.the wife said Phil,the Dr. cant even hang a picture. What I'm saying some need time to learn fundamentals. Its maybe easy for a mechanical inclined person. I'm just trying encourage understanding and patience. We need everyone in this Pontiac hobby.

[PAUL K]

One could do all kinds of measuring with plastigage and it'd certainly be better than nothing. The problem is, it's the actual housing bore that needs the be measured. The bearing will want to retain it designed form. The housing bore supports the bearing and if that's not the correct shape (let's say too big) it will not support the bearing under load. The bearing will move around and fail.

In other words, let's say the main cap is .003 too big. When you torque the bearing up it might measure fine because the bearing retains its natural shape. When you put a load on that bearing it'll have room to move in the cap and the hydraulic support will fail.

Obviously if the new cap fits to tight you'll know right away.

The number five bearing has the most load on it. It'll fail in short order if things aren't correct.

[NeighborsComplaint]

Since all pontiac blocks of that era were factory align-honed, wouldn’t the caps always be concentric to the crankshaft centerline and centered (within OEM tolerances) on the main cap/block saddle parting line?

[hurryinhoosier62]

Originally Posted by NeighborsComplaint

We're not talking obscure parts interchangeability issues, this is fundamental machine parts assembly and plain common sense.

We all have Smartphones these days and accepted methods of disassembly and reassembly (even if generalized) are right in the palm of your hand. Google "How do you seat a main bearing cap on a Pontiac V8?" and here is what you get:

"Seat the main bearing caps by carefully aligning, lightly tapping, and snugging them before torqueing in sequence, ensuring proper clearance and thrust bearing endplay."

Before Smartphones, you bought a Chilton's Manual to consult.

How many times have you personally replaced the main cap on a block, then align bored it? Me? Couple of thousand times. There are ALWAYS fitment issues; that is why I kept around 150-200 main caps set for the most common blocks we machined. On average, I was able to match a replacement main cap after 8-10 tries. Keep in mind this was a cap that fit close enough to cut the cap( usually .006-.010) so the carbide of the align bore machine was taking the majority of the material out of the cap instead of the block. Then, you have to resize the rear main seal area to prevent fitment issues. Here is my advice to everyone opining here( with the except of Mike G): if you have never performed this operation on an engine block leave it to the professionals who perform these matching operations every day.

[hurryinhoosier62]

Originally Posted by NeighborsComplaint

Since all pontiac blocks of that era were factory align-honed, wouldn’t the caps always be concentric to the crankshaft centerline and centered (within OEM tolerances) on the main cap/block saddle parting line?

Cite your CREDIBLE sourcing that PMD align honed all the V-8 they used. Of all the virgin Pontiac blocks I have torn down I have never seen one with crosshatch on the main saddle.

[mgarblik]

I agree. I think OE blocks were just line bored, not honed. They did a pretty darn nice job too. A production line hone operation would add allot of cost to building an OE engine. A boring, cutting operation would require much less maintenance on the assembly line. (cutter vs stones). Agree with others that just installing a random, used cap from another block, even if it looks ok with plastigage is a gamble. I think you would sleep better if it was verified with a bore gauge and fixed if needed.

[steve25]

The factory standard production rod bores are not honed either, but don’t know about the out sourced rods like the SD455 and the RAV type rods .

[NeighborsComplaint]

Originally Posted by hurryinhoosier62

Cite your CREDIBLE sourcing that PMD align honed all the V-8 they used. Of all the virgin Pontiac blocks I have torn down I have never seen one with crosshatch on the main saddle.

Check the post right below yours.

[NeighborsComplaint]

Originally Posted by hurryinhoosier62

How many times have you personally replaced the main cap on a block, then align bored it? Me? Couple of thousand times. There are ALWAYS fitment issues; that is why I kept around 150-200 main caps set for the most common blocks we machined. On average, I was able to match a replacement main cap after 8-10 tries. Keep in mind this was a cap that fit close enough to cut the cap( usually .006-.010) so the carbide of the align bore machine was taking the majority of the material out of the cap instead of the block. Then, you have to resize the rear main seal area to prevent fitment issues. Here is my advice to everyone opining here( with the except of Mike G): if you have never performed this operation on an engine block leave it to the professionals who perform these matching operations every day.

Hey good for you. I'm surprised your big head fit through the forum door.

In a perfect world, with motor out and at the machine shop, of course you would have the motor align-bored or align-honed depending on the severity of the misalignment of the replacement cap. The cost and effort is insignificant vs the potential for main bearing failure..

The OP is looking for an option short of pulling the motor out, disassembling and hauling it to the machine shop, but you obviously missed the drift because you know it all.

He is willing to risk the consequences of a short-lived rear main bearing if the "fast-pass" doesn't work out. The inspection procedure I suggested, removes as many variables as possible for a successful outcome in light of the circumstances and his desire to avoid a full teardown.

[NeighborsComplaint]

Originally Posted by hurryinhoosier62

Cite your CREDIBLE sourcing that PMD align honed all the V-8 they used. Of all the virgin Pontiac blocks I have torn down I have never seen one with crosshatch on the main saddle.

Right here on this forum, baby. Call it align boring if you prefer.


"Pontiac V8 engine blocks from the 1960s through the 1970s were align bored at the factory as part of the casting and assembly process. This means the main journal saddles were honed to a uniform size and alignment before the caps were machined, ensuring proper deck height and main bearing fitment."
Source: forums.maxperformanceinc.com."

"Factory alignment boring was standard for most Pontiac V8 blocks during this period. The block was cast at Saginaw Metal Casting Operations and then assembled at Tonawanda Engine before delivery to Pontiac Assembly."
Source: forums.maxperformanceinc.com


"The process involved honing the main saddles to a precise diameter and ensuring they were in perfect alignment. This was done before machining the caps, so the caps could be cut flat and seated without distortion."
Source: forums.maxperformanceinc.com

"If a block arrived at the factory with misalignment or out-of-size saddles, it would be align honed (a manual honing process) to correct the issue before further machining."
Source: forums.maxperformanceinc.com