SBM 340 Rear Main Cap

[ironmonger]

Thank you everyone. Yes, this is ongoing. I should have this resolved next week. Lots of great advice.
I'll post more when this issue is closed.

I also bought a 74 d100 club cab over the weekend!
My next dig is a wiring diagram when the weather improves.

This is the conversation I had this morning:

I'll follow up with Kent tomorrow.

I went to my shop, I asked if they could grab the end cap I had brought in. With it on the counter, I asked, which way is it "too short" and i displayed length, width and height with my hands.
His response was the distance from the outer edge to the lip of the crescent shaped channel was too short..... and of course, the other side was too long.
What he was explaining was the cap was not centered to the same axis as the block. He explained that every cap would be offcenter as they were matched to another block.

I don't think they're really comfortable with Mopar so I told them I would take the block elsewhere and just bring them GM in the future.

If I'm correct, isn't the correct way to fix this is to weld thecrank journal and rebore it?

 

[ironmonger]

I have broken into two parts the 318 main caps with only
400 HP on our stocker.

I do not advise them for anything very High perf.

375 is my target

 

[GTX JOHN]

That is still more than I would ever use them ever again.

340 Main caps are not expensive and that hard to find.

The thing about an incorrect axis and wrong sizing is new to me?
Align bore has always cured any issues for me without that drama.

If it is a $500/$1000. engine you are not risking a lot except
you time. If it is a significantly high $$$ build then the risk and
expenses goes up if you have a catastrophic failure.

Just my .02 = For Free!

 

[ironmonger]

That is still more than I would ever use them ever again.

340 Main caps are not expensive and that hard to find.

The thing about an incorrect axis and wrong sizing is new to me?
Align bore has always cured any issues for me without that drama.

If it is a $500/$1000. engine you are not risking a lot except
you time. If it is a significantly high $$$ build then the risk and
expenses goes up if you have a catastrophic failure.

Just my .02 = For Free!

I think they're just making excuses.

 

[gzig5]

This is the conversation I had this morning:

I'll follow up with Kent tomorrow.

I went to my shop, I asked if they could grab the end cap I had brought in. With it on the counter, I asked, which way is it "too short" and i displayed length, width and height with my hands.
His response was the distance from the outer edge to the lip of the crescent shaped channel was too short..... and of course, the other side was too long.
What he was explaining was the cap was not centered to the same axis as the block. He explained that every cap would be offcenter as they were matched to another block.

I don't think they're really comfortable with Mopar so I told them I would take the block elsewhere and just bring them GM in the future.

If I'm correct, isn't the correct way to fix this is to weld thecrank journal and rebore it?

I think what he's saying is that a set of caps that are not original to the block will not line up correctly with the existing crank bore line. This is true. Any replacement cap is 99% likely to need to be line bored and then honed in place. This is done by removing a few thousandths off the flat face of the cap (enough to accommodate the mismatch), mounting them in place, rough line boring the crank journals, then finish honing the journals. Process is much like that used to resize the big end on a connecting rod, just bigger scale and all done in one operation. Another thing that can throw a wrench in the works is that the tenon on the replacement cap properly fits the female tenon in the block. If it is too big, that's good because it can be thinned down. If it is too narrow and the cap can move side to side, it is a more difficult fix to use that cap. The fit needs to be tight, line on line or slight press fit so the crank can't walk side to side under load.
Not every shop is setup to line bore, while most seem to have a line hone bar. I had a heck of a time finding a shop that was capable and willing to fit replacement caps to my extra 340 block. There is additional risk in doing this in that the center to center distance of the crank to cam can be affected if done poorly. If they aren't enthusiastic about doing the work, find another shop.

 

[ironmonger]

Awesome reply, ty
Yeah, I think the problem is they're uncomfortable doing a line bore. I decided over the weekend, if they're less than enthusiastic, I don't want them making the attempt.
The tenon isn't something I've considered. I'll look at that.
I wish they had said 1 month ago, "we're not comfortable doing a line honing."

 

[Newbomb Turk]

This is the conversation I had this morning:

I'll follow up with Kent tomorrow.

I went to my shop, I asked if they could grab the end cap I had brought in. With it on the counter, I asked, which way is it "too short" and i displayed length, width and height with my hands.
His response was the distance from the outer edge to the lip of the crescent shaped channel was too short..... and of course, the other side was too long.
What he was explaining was the cap was not centered to the same axis as the block. He explained that every cap would be offcenter as they were matched to another block.

I don't think they're really comfortable with Mopar so I told them I would take the block elsewhere and just bring them GM in the future.

If I'm correct, isn't the correct way to fix this is to weld thecrank journal and rebore it?

Ok, that makes sense to me. You can’t just throw any old cap on it and expect it to fit.

Even the first four caps might not fit.

That’s why the Ritter cap is the best deal.

It’s already machined for the seal and all that. And the hole (or half hole) in the cap is undersized so you can machine it to fit.

Sounds to me like the guy knows what he’s talking about.

 

[Newbomb Turk]

I think what he's saying is that a set of caps that are not original to the block will not line up correctly with the existing crank bore line. This is true. Any replacement cap is 99% likely to need to be line bored and then honed in place. This is done by removing a few thousandths off the flat face of the cap (enough to accommodate the mismatch), mounting them in place, rough line boring the crank journals, then finish honing the journals. Process is much like that used to resize the big end on a connecting rod, just bigger scale and all done in one operation. Another thing that can throw a wrench in the works is that the tenon on the replacement cap properly fits the female tenon in the block. If it is too big, that's good because it can be thinned down. If it is too narrow and the cap can move side to side, it is a more difficult fix to use that cap. The fit needs to be tight, line on line or slight press fit so the crank can't walk side to side under load.
Not every shop is setup to line bore, while most seem to have a line hone bar. I had a heck of a time finding a shop that was capable and willing to fit replacement caps to my extra 340 block. There is additional risk in doing this in that the center to center distance of the crank to cam can be affected if done poorly. If they aren't enthusiastic about doing the work, find another shop.

EXACTLY^^^^. I got treed. Again.

But that’s the fact.