T-Series Question about regrinding a TD crank

[Sarastro]

My TD was run by a PO without oil pressure. I've dug into the engine, and I have a question about the crank.

The rod journals are pretty well chewed up, so I clearly need to get them reground. Currently, the journals are 0.020" under, so there's plenty of room to grind them some more.

The problem is the main bearing journals, which are now 0.040" under. According to the shop manual, the maximum grind is 0.040". However, I noticed that bearings of 0.050" and even 0.060" are available, which puzzles me a bit.

So,

1. Is the 0.040" limit for the mains really valid? Can you really grind it 0.050 or 0.060 under, or is that a bad idea?

2. If so, what do you do if it needs to be reground further? Can the crank be rehardened, or is this one of those things that is possible in theory but not in practice?

3. How do I tell if the journals really need regrinding? The front and center don't look bad to me, but the rear one has a clear score line around it, maybe a couple mils deep, just enough to be visible and feel it with a fingernail. It's actually in the center, so it's under the oil groove, not really in the bearing surface.

I've attached a picture of the rear journal, although I realize it's hard to evaluate a crank just from a picture like this.

 

[Bob Claffie]

First off, I am NOT a professional machinist nor an engine builder. That being said, IMHO I would grind the rods and leave the mains alone. If that picture represents the worst case journal you have no problem there. Possibly the crank man will suggest a polish job, that should be enough. Bob

 

[glemon]

I would echo exactly what Bob said above, including the part about not being a professional, but if you mains are in spec get new main bearings, and call them good with a polish from the machinist

 

[LarryK]

Depends on how the car will be driven and to what extent you want the engine to keep it's strength. The rod journals are chewed. Did the rod ends themselves get hot enough to distort? Have the mains gotten hot enough to distort. You only show a bare crank journal, what is the coloring on the bad bearings. You are going to put some money in making it right might as well do the rod and main journal distortion measurements and correct. I assume the PO knew what happened and discounted appropiately so you really aren't out that much more for a build up and regrind to specs, then know you can run a club tour or a group trip without worry. Besides you now have to wonder if there is build-up of bearing sludge in the oilways.

 

[PAUL161]

The only thing I could say is, T series cranks have a habit of breaking around the front main journal. I would have a professional look at the crank and give his advice whether it can be reconditioned and made safe. Crank journals can be hardened, the internal strength of the crank is questionable. A new crank is cheaper than a new block plus a new crank, possible cam etc. They ain't pretty when they come apart! I would NOT cheat in this area! JMHO. PJ

Click on this Moss link and read about grinding T series cranks. PJ

https://www.mossmotors.com/graphics/products/PDF/451-490.pdf

 

[Sarastro]

Good point about the rods and distortion; there was nothing obvious, but I'll look into it more deeply. Interestingly, the top end is still fine. Compressions were really good, and I ran a bore gauge down the cylinders; they show no taper or out of round within a half mil. Interesting that the cylinders were unhurt, but the crank was badly damaged. The camshaft is in bad shape, but just from ordinary wear on top of previous regrinds. I'm replacing it.

Also, I really should measure the main journal diameters. One of the rod journals looked good, for example, but it was 3 mils undersized. Might have been a bad previous grinding job, but one never knows...

I hadn't thought about simply polishing the main journals. I suspect that might be enough.

As for use, I really don't think I'll be driving this car much, so it won't get any great stress. I really don't want to replace the crank unless clearly necessary; they are scary expensive. Of course, I'll get it magnafluxed when I take it in for grinding, and if it's cracked, well, that answers that...!

Anyway, thanks all for your thoughts. It really helps a lot.

 

[Sarastro]

Hmmmm....just checked the crank carefully this time, and the mains are actually ~0.035" under (2.0105-2.0110"; original diameter is 2.0470). So, there must have been 0.035 bearings available at one point, but I don't think they still are. So, looks like we have to go to 0.040, which should be plenty to get rid of any minor scoring and smooth it out well. Whew!

 

[Bob Claffie]

As Larry mentioned above about the color of the rod big ends. If they are blue there was probably enough heat to warp them. Relatively simple and inexpensive to get the rods reconditioned. No sense putting oval rods back on a round crank. Been there, done that. Bob

 

[LarryK]

An older 4 cylinder does run quite a few rpms per mile. Better check valves and lifters if cam is gone. No oil pressure does take it's toal, can't be cheap here and keep value of car up.

 

[Sarastro]

Fortunately, the valves, rockers, and lifters look good. Funny that the camshaft lobes had a lot of pitting, but no scoring, and the lifters were perfect. Seems that just the crank and bearings were damaged, although it seems strange to me that the upper end stayed OK. The cause of the problem is clear, in any case: the oil pump drive gear on the camshaft broke, and probably the mating gear on the pump jammed in it and broke, too. The PO replaced the pump, and it actually worked, for a while, but I'm sure that eventually it would have jammed again. Anyway, I have a new camshaft on order, and I'll take the crankshaft to the shop. I can measure the rods here, and if questionable, they will go too.

 

[Bob Claffie]

With all the hardness gone from youe crank due to the previous grindings, you might ask your machinist about "Nitrating". This is a surface hardening procedure. This only gives a few mils of protection but you need all the help you can get. It's not done as a routine and usually has to be done by a special.ist shop. Not cheap but the alternatives aren't either. Bob

 

[Sarastro]

I took the crank to a grinder in Los Angeles on Thursday, and he thinks he can do it within the shop-manual limits. If that's the case, I can't see why any further hardening would be necessary. The place I used only does grinding. Even so, nitiriding is worth considering; I'd have to find someplace else to nitride it, but I'm sure there's something, somewhere in this town. You can get anything else, whether or not it's legal or good for you....!

Meanwhile, I received my replacement camshaft. It's a reground cam, very pretty. Making progress!

 

[drooartz]

Progress is good. :thumbsup:

Nothing like the the arrival of new parts to make you smile.

 

[Brinkerhoff]

I took the crank to a grinder in Los Angeles on Thursday, and he thinks he can do it within the shop-manual limits. If that's the case, I can't see why any further hardening would be necessary. The place I used only does grinding. Even so, nitiriding is worth considering; I'd have to find someplace else to nitride it, but I'm sure there's something, somewhere in this town. You can get anything else, whether or not it's legal or good for you....!

Meanwhile, I received my replacement camshaft. It's a reground cam, very pretty. Making progress!

I think the nitriding needs to be done before the journals are ground. Can you buy bearings for the resized crank? Though expensive, replacing the crank might be the smartest thing to do at this point.

 

[Bob Claffie]

Another random thought. Are the cranks forged or cast ? If forged the "extra" hardening may be unnecessary. One the other hand if the original is a cast crank I would certainly harden it. To the above post: nitriting is only a few thousandths deep. If you harden before machining you would machine off all the "new" hardness even if the crank was only cut .010. Must be done after as it does not add anything to the diameter. Bob

 

[PAUL161]

Another random thought. Are the cranks forged or cast ? If forged the "extra" hardening may be unnecessary. One the other hand if the original is a cast crank I would certainly harden it. To the above post: nitriting is only a few thousandths deep. If you harden before machining you would machine off all the "new" hardness even if the crank was only cut .010. Must be done after as it does not add anything to the diameter. Bob

Nitriding is done only after the crank is totally finished! When it's ready to drop in the block, not before! PJ


A quick note,

Cast vs. Forged
A crankshaft is a pretty substantial chunk of metal arranged in a highly contorted shape. There are a couple of different ways to arrive at the basic shape, and this forms the basis of whether the crank is a forged or cast piece. In casting, a mold is made and molten crank material, usually cast iron, is simply poured in to create the raw casting. Casting is cheap, the tooling is long lasting, and the raw casting springs from the mold very close to the required final shape, minimizing the final machining requirements. All of these attributes are endearing enough to make cast cranks the overwhelming favorite for OEM and mild performance applications.
In creating a forged crank, an entirely different process of metal forming is used, aptly referred to as the forging process. In forging, a hot chunk of rolled steel is placed between heavy dies having the pattern of a crankshaft. Under extreme pressure supplied by a forging press, the metal is squeezed into the crank's basic shape. The simplest crank forging dies are arranged in a single plane, which produces a crank forging that has all the crankpins in one plane. To index the crank throws at 90 degrees, the raw forging is twisted to offset the journals in two planes to create the final raw crank blank.
An improved forging process involves forging the crank in two planes, so that all the journals are pressed into their final configuration, eliminating the need to twist the crank to index the journals. The result is fewer internal stresses in the forging, as well as an improved grain flow in the metal. Cranks made with this type of tooling are referred to as non-twist forgings. Tooling for a non-twist forging is considerably more complex and less durable than that for a simple flat forging, and there is typically more excess material to be machined from such a blank to create a finished crankshaft. Manufacturers producing crank forgings in huge volumes naturally gravitated to the lower cost and higher tooling life of a flat forging. In the aftermarket, with smaller production runs and an emphasis on durability for high-end cranks, non-twist forgings are available for many popular engines.

 

[Sarastro]

I suspect that MG did whatever was simplest and cheapest, which means that the cranks are cast. They look like a casting, to me, for what that's worth.

I got the crank back; it looks pretty good. The rod journals are 0.050" under, which is the limit. The mains, as it happens, are 0.020" under. I had measured them to be more than this, but I now realize I was using my 2" micrometer, which has no 0.025" cal lines beyond the 2" point, so what I thought was 2.012 was really 2.037. Duh!

Anyway, there's no way at this point that I can shell out $2500-$3000 or so for a new crank, so this one will have to do. As long as it's within the factory specs for regrinding, I don't think I'll have a problem. Nitriding may still be an option, but I've been busy with other things and haven't looked into it.

I loved the crank-guy's shop. A really creepy, dark place in south LA with a hand-lettered sign over the door. Full of lathes and other equipment, with cranks piled everywhere. Apparently he is one of the main places in LA where shops that don't do their own grinding send their cranks.

I'm still waiting for some engine parts, so I've been working on the body. I started a web site for the restoration, in case anyone is so low on entertainment that he'd like to follow along:

https://www.nonlintec.com/mgtd/

Thanks for all your suggestions!

 

[Roger]

I'm not certain, but very much doubt that the X-series engines had cast cranks. Certainly the pre-war OHC engines had forged cranks, as did pretty well every other manufacturer. Not nearly enough was known about casting steel back then!

 

[Hap Waldrop]

The only classic MG engines I know that ever used a cast crank was the last of the MGB engines. On a steel crank , it is possible to build back the journals with welding, but not cheap.

 

[Sarastro]

Hmmm...interesting. I would have thought that casting, which has been around since the Bronze Age, would have been the simpler and more mature technology. I imagine that forging provides some advantages in hardness, though, and maybe that's one reason (of many, probably) for doing it.

I wasn't aware that the journals can be rebuilt. Even if it's expensive, probably anything is cheaper than buying a new crank for this engine. They're scary expensive.