[ThreeNuts]

Not a good day. Not feeling well; should probably have stayed in bed.

I have a '66 Bonneville convertible I've owned since 2000. Matching numbers, well-cared-for. In 2010 I pulled the engine & sent it to a local shop in Berlin, NJ to be rebuilt.

Within three years, the rear main seal began leaking. I lived with it; it kept getting worse until I decided this past fall that I had to pull the engine to replace the seal.

I did that today, installing a Viton 2-piece since I wasn't pulling the crank. When I torqued down the cap, I neglected to tap it down in place first. Cracked it.

I have another '66 389 in the loft of my garage; my understanding is that no matter what I do, the engine essentially will have to be rebuilt. I need to find a competent shop to do this and also determine if I can get another cap and have it cut to fit or have the other engine rebuilt.

[SRGN]

Not going to need a full rebuild, but you will have to tear it down for a line bore with the replacement main cap.

[ThreeNuts]

Thank you! I have a line a replacement cap.

Now I need to find a shop...the place I had it rebuilt is no longer in business. Open to suggestions.

[SRGN]

You mentioned NJ, so if you get it all torn down, I would send the block and caps to Weston machine in Piscataway. Charlie is a very good machinist, not cheap but worth the money. I've been sending blocks and cranks to him for years, no disappointments.

[Dragncar]

Don't do this, but as teenagers my brother had a 307 Chevy in his 69 C10. Found a cracked rear cap and he just stuck one on from another engine we had laying around. Ran fine for all we knew. Truck made it from far Nor Cal to Arizona when he left for school.

[steve25]

You never no, that replacement rear cap you have coming might fall within an acceptable range of being round to just go with it.

First even at home you can carefully determine if the crank will even spin with that new cap bolted on.

Lay your oiled up crank in on the bearings you have just with that rear cap and number 1 cap.
Do not add caps 2,3 or 4.

Sung down the number 1 cap while rocking the crank back & forth sung down the rear main bolts little by little.

Of course if just the action of sitting that rear cap on binds up the crank then for sure the block must be align bored.

Even If it does not bind up then take it to a shop anyway to see what the rear main bore actually checks in at.

You never know the Gods might smile down on you and you can run it as is and not loose much drive time now that spring seems to finally be here.

While the crank is at the shop have them polish up where the oil seal rides to remove most of the searations so the new seal will live.


Also in terms of rear main oil seals I would run the one piece one as that makes for one less potential leak point.

[mgarblik]

As you said, it's not a good day when something like this happens. As bad things go, it's not as bad as say a Chevy with the same issue. Since a Pontiac takes it's thrust on #4, the only thing associated with the #5 is the seal groove and the diameter. If it's possible to get the block to a "Pontiac shop", that has a bunch of main caps laying around, it's certainly possible to find one that will fit close enough so it will only need a line hone, rather than a line bore and hone. That would save you a bunch of time and money. When you get into line boring, the machine shop will need a dedicated line bore machine. Most shops have a line bore attachment for their line hone and they will not bore the front or rear cap. They are made to bore #2,3,4 mains only for people adding billet caps in the middle of the block or converting to 4-bolt mains. A dedicated line bore job around here is a $500.00 operation by itself. Then the line hone for another $300-400. Than can get expensive if not needed. In my experience, if you had 5-6 donor rear main caps, there is a good chance one of them would be close enough to just cut and line hone. Could you post a pic of the cracked cap? if cracked just at the dowel hole, they can be cleaned up and reused on a stock rebuild. Especially the rear one. That's a big, strong cap. Good luck with it.

[sdbob]

That's what I was wondering. If cap could be repaired and used.

[ThreeNuts]

Appreciate all of the responses and advice.

I do not have the room or the proper setup to tear the engine down to the short block. I have to take it to a shop. I am prepared to absorb the cost of a total rebuild should that be necessary, so anything less than that is good. I assume it would be at least double what I paid in 2010, which was $4500.00. It's bored 040 over. There isn't 20,000 miles in it in that time.

I just ordered a set of caps off of eBay as they seem to be hard to find.

I do have a second engine up in storage I could pull the rear cap from but doubt either will fit straight-up without a rebore.





Don't think there's any repair possible for this.

[John Milner]

I had a cracked rear main cap on a 428 of mine. A friend gave me a rear cap and I had a machine shop install the cap to the block. They ground the new cap, pulled the dowel pins, centered the cap, drilled for bigger pins and line hoaned. It worked perfectly with a rope seal. Total cost was about $300 to repair it.

[mgarblik]

Thanks for the pic. Yea, no fixing that poor cap. Having a cap fit, pins removed, and upsized, plus a line hone would never happen around here for $300.00 Maybe 15 years ago! That's a solid 6 hour job without automated equipment I don't know of any machine shops working for $50.00 an hour. We are about $100.00 an hour for very average work. $125-130 for a shop that knows what they are doing.

[Mike Davis]

You may have dodged a bullet....I have never seen a rear main on a Pontiac crack like that and I have taken a ton apart. You may have already had a small crack starting and when you tightened it down, it may have finished it off. Find a shop that knows Pontiacs like stated and see if they can find one close so all you need is a touch with a line hone.

[Scarebird]

Originally Posted by John Milner

I had a cracked rear main cap on a 428 of mine. A friend gave me a rear cap and I had a machine shop install the cap to the block. They ground the new cap, pulled the dowel pins, centered the cap, drilled for bigger pins and line hoaned. It worked perfectly with a rope seal. Total cost was about $300 to repair it.

When this replacement cap arrives, could one find out what the clearance is, remove the dowels and use shim stock that thickness to center the crank and torque down the cap. Then ream out for the next size up dowel?

[sdbob]

From your picture I've never seen one do that. I use find all the 455s I could ,strip them out and give them$$ to an SS racer running out of Arnold Hines shop. All the Pontiacs engines I've assembled for myself 350,389,421,455 ,SD455 I've never encountered that.wow.

[ThreeNuts]

Originally Posted by sdbob

From your picture I've never seen one do that. I use find all the 455s I could ,strip them out and give them$$ to an SS racer running out of Arnold Hines shop. All the Pontiacs engines I've assembled for myself 350,389,421,455 ,SD455 I've never encountered that.wow.

That's because anyone else doing this would have had the sense and wherewithal to tap the damned thing down before dropping in the bolts and putting a torque wrench dialled to 120-lbs on it.

I am assuming that a line hone means the crank is coming out. Also therefore that the pistons are coming out. Which means the intake, heads etc are also coming off.

I fantasized about a shop pulling the crank while leaving the pistons in the cylinders. Serious doubts about how that would work.

[ThreeNuts]

Originally Posted by mgarblik

Thanks for the pic. Yea, no fixing that poor cap. Having a cap fit, pins removed, and upsized, plus a line hone would never happen around here for $300.00 Maybe 15 years ago! That's a solid 6 hour job without automated equipment I don't know of any machine shops working for $50.00 an hour. We are about $100.00 an hour for very average work. $125-130 for a shop that knows what they are doing.

I'm staring at a likely teardown & rebuild, at least all new bearings & seals - (odd that the rebuild shop used a 2-piece seal in 2010) - maybe the pistons & cylinders are good...they should be, with less than 20K on it, biannual oil changes, and no other issues...it runs (ran) great...

...So anything less than $4K is fine, I'm more concerned with finding a competent shop within 100-miles I can take it to. I have a pickup so delivery & pick-up are not the issue.

[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. Install the bearing half dry in the new cap and install it on the motor. Use light taps with a dead-blow hammer to seat the cap and torque the bolts to spec.

Spin the crank slowly with a toque wrench and note and binding by turning in 1/4 turn revolutions. Run through a couple revolutions. Your torque wrench and the resistance you feel will tell you if the crank is tight from misalignment.

Remove the main cap and look for any contact pattern that confirms misalignment (blueing rubbed off in an even band). If all is good, clean off the blue, libe the bearing and reassemble.

Also, take care installing the old bearing in the new cap. It may not press right in as it may have spread slightly when the cap broke but not badly enough that a couple light taps on the top of the cap with the mating surface on flat sheet steel wouldn’t seat it.

It’s not perfect but it could save you a rebuild and potentially run a long time.

[Scarebird]

Originally Posted by NeighborsComplaint

...It’s not perfect but it could save you a rebuild and potentially run a long time.

My thoughts exactly. You really have nothing to lose here as if it starts acting up you will hear it in time.

[Cammer-6]

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

[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. I remember putting that 389 in my 55 with a slant pan hydro. Everything bolted up fairly easy. Fired the new 389 right up. Oil POURING out of the front of the hydro. I thought seal ,nope . The 389 has a slight bump on the rear flange that the 287 didn't. Flywheel wobbled, Took the front pump out of
the trans. Now what? I happened to have a B&M catalog ,yep they have special machined flywheel for this very thing. Irony I had 2 flywheels when assembling 389 to hydro. One was cleaner so I used that one. The dirtier one was machined to fit the 389 in parts I had bought. Ugh. Live and fall down get back up fall down again! That's the way I learned mistakes then asking questions.