General Tech Crank bearing cap broke...Torque??

[TR3driver]

Geo, think about what happens when a cylinder fires. The piston is forced downwards, the rod carries the force to the crank throw. But for the crank to turn (instead of just falling out the bottom), something has to push the other way on a main journal. Main bearings take plenty of force (tho not as much as rod bearings).

In spite of the hype, JB Weld is not even as strong as many plastics. It's a good product, but calling it "steel" is grossly misleading.

My guess is that one or both of the inserts was the wrong thickness for that crank journal.

 

[TR4nut]

If you do opt for a replacement block, at $125 that 73 block doesn't look too bad to me, at least before checking the cylinder condition and the 76 parts will fit.

 

[Geo Hahn]

Geo, think about what happens when a cylinder fires...

Thanks for the explanation. I was a Liberal Arts major, I should probably stick to problems concerning Western Civ or the works of Henry James.

 

[rooster]

I heard back from the engine block person and got a better photo. To my untrained eye it looks like it might require some machining and or honing to get it to a usable state:


If that is the case I think I'm better off to stick with my current block.

Anyhos, I welcome expert opinions on the condition of the one in the photo, maybe it's a diamond in the rough?

@Marvin, do you have a used cap for the back bearing on a TR6?

 

[rooster]

Oh, I also have gotten a couple of quotes on a line bore, starting at 300 to 500. Both places wanted to take a look at it first.

Around here the go to engines are the American V8's and most places are equipped to handle them, the British engines are not as popular and there are not many places that have the tooling for them.

 

[poolboy]

I think I'd at least try a replacement cap.
Put the bearings back in, put a line of green Plastigage in a a couple of places between the lower bearing and crankshaft journal and little by little torque it down..
Then pull it off and check the plastigage. If the results are normal, reassemble, make sure the crank turns freely and call it a day.

 

[CJD]

Oh, I also have gotten a couple of quotes on a line bore, starting at 300 to 500. Both places wanted to take a look at it first.

Around here the go to engines are the American V8's and most places are equipped to handle them, the British engines are not as popular and there are not many places that have the tooling for them.

That’s about normal. Any shop with the line boring machine can do it. Many don’t want to mess with British engines, so they play stupid. I have a machinist I have used for 30 years here in Fort Worth. Great guy, but he bitches my ears off every time I bring him the British parts...but after sweet talking him for a bit, he does it for me.

It sounds like the math leans towards keeping your block. If you can find a replacement cap and get it bored for $600 total, it saves having to buy and transport an unknown quantity. The motor in the pic has obviously not run this millinium. Always best to stick with the block you know unless a “fresher” and reasonable short block shows up soon. At least one that will turn over...which I doubt the one in the pic will.

 

[CJD]

Oh, have you removed the head yet? When you do, scrape your fingernail upward against the ridge at the top of the cylinders. If your nail catches on the ridge, then your current block is due for an oversize cylinder bore while it’s apart. If your fingernail passes over the ridge without catching, then you can get by with new rings and a hone.

If it turns out your current block will need cylinder boring...that means new pistons too. Something to think about, and could possibly tilt the math towards the old short block you found?!? It will definitely need the cylinder bore with oversize pistons too...but it will not need the line bore on the mains.

 

[rooster]

Hi John,

I don't have the head off yet. It's stuck on. I've got all but one stud out. I am giving the stud a soak with 50/50 atf and acetone, and try it again in a few days. I was going to try the rope trick to get it off.

@everyone
They guy with the block has a complete engines for sale as well. He wants 500 for it. I'm inclined to pass on this and give Poolboy's suggestion a try. You never know I might get lucky and it will work. I just need to get a hold of a bearing block.

 

[malbaby]

In addition to advice already posted, you may be lucky and not have to line bore the crank tunnel...I would remove the crank and using a good straight edge, check to see if the block main bearing seats are exactly level....If OK, then replace all the bearings and caps except the rear main cap....then gradually tighten all the caps [not to full torque]...if the crank still turns OK that is a good start...Try and get a few used rear main bearing caps and repeat the procedure, checking for crank movement as you tighten.

 

[Marvin Gruber]

Will dig into my stuff tomorrow. I was letting everyone speak first. I would try a used one first with plastaguage, odds are it will work. It is the rear one you need?

Marv

 

[CJD]

There is more going on with a main bearing cap than most seem to understand. In a perfect world, the caps are all installed on the block at the factory, and the mains are bored together perfectly on center vertically and horizontally. In the real world, if the bore is off by a fraction of a degree, or just one or two thousandths...which would be a GOOD block!!, then these are the issues with just putting in a different cap:

1). The “crush” is what holds the bearing insert in place AND keeps it from spinning. If the bore is just .002” up or down at the rear cap, you could lose the insert crush, causing a spun insert or a reduced bearing clearance as the insert is over-crushed. Worst case you could crack the new cap with the over-crush stress. The little tabs on an insert will not prevent an insert from spinning. Those are for alignment only...crush is what keeps the insert in place.

2). If the bore was just .001” left or right of center, then the inserts will have a lip where they meet. This will not show with plastigage. On one side the insert lip will act to scrape the oil from the crank AND reduce the true bearing clearance. Double whammy.

3). There is a good chance the bore is slightly off in both vertical and horizontal directions...resulting in multiple problems described above. Remember the replacement cap was also bored in an imperfect world...doubling your chance of mis-match problems.

4). If the locating tabs are off by a thousandth between the original cap and the replacement cap, then the replacement cap could have a “twist” in relation to the crank centerline. This misalign would also not be evident with plastigage or mics, but could eliminate all clearance on the insert edges.

My descriptions are for only 1-2 thousands bore error. Once again, that would be a good factory bore job. I have seen them off by .005”, which would be twice the entire recommended bearing clearance. It’s your engine and your decision. But it’s one of those items that if it goes wrong...it will cause a lot of damage. Is it really worth it?

 

[rooster]

Hi Marv,

Yes it's the rear one, from a TR6.

Jon, I am thinking that back in the day when these were manufactured there was probably a lot of variation from one day to another, or even one block to another depending on how they had the machining set up, the mood of the operator, time of day, temperature etc. etc. I don't expect the replacement cap to be a simple bolt in fit. It's almost guaranteed to require some adjustment (line bore) to get proper alignment. I will for sure be taking the block to a machine shop and getting it done.

 

[TFB]

I think re line boring would require that all the main saddles and caps be machined flat and thus create an undersized hole to line bore.
Wont this also move the crank center up?Not sure of potential problems there.Maybe not much if it only changes a few thousands.
But with all the variables unless you have a machine shop and machinist that is willing and capable I think I would,probably for cost alone,source another block.
Tom

 

[charleyf]

As to getting a replacement cap---. I had a TR3 block that was missing all the caps. I had another block so tried to fit the caps from that block to the other. I could not even get the cap to seat in the space provided. So I am thinking that unless you find a cap from an engine very near yours in engine number , you are not going to do well with your fitting. And this is before you even try to set the the crankshaft in place.
Charley

 

[sp53]

https://seattle.craigslist.org/oly/pts/d/1976-triumph-tr6/6365681918.html
maybe this

 

[Marvin Gruber]

Rooster,
No luck on the rear main. I have several complete engines and blocks left but would hate to tear one down for a cap that may or may not work. Go back to the guy with the $125 block. Tear it down and see what you have. If crank is good you can recoup your money on it alone.
Marv

 

[rooster]

Hi Marvin,

Thanks for checking. I think I've found one that might work locally. There is a local guy who has some of this kind of thing lying around, and he's willing to sell me one.

The short block in Seattle looks like it would be a good candidate, if I need to go that way.