I'm working on a '86 Z24 w/96K on it. I'M rebuilding it. It suffered a blown head gasket. The block and head were both ruined (actually I had the head welded on and it may be saveable). The two cylinders were passing gas to each other and it eroded a pretty good chunk out of both of em.

So, it is a low mileage motor that never had problems w/oil delivery.

I have the crank out and the machinist miked it today. The tolerance is a span of .0005 (I think).

The rod journals are .0001 from being to big, or .0004 from the low end (I think, I may have an extra zero in there). They say that if they were to grind the crank, on the rods, that is the tolerance that they'd be shooting for and that it has good specs, on the rods.

Now, the mains are a dif story, they are w/in specs, but they are .001 or .0001 from being to small. Again the tolerance is a span of .0005 (or .005) and they are at the small end. So, if the minimum is 2.50, it is at 2.501 or 2.5001 and 2.505 or 2.5005 is the max. diameter.

Grind the mains or no???

Grind it.

With all the work you will be doing, why skimp there. The main and rod journals are critical to building oil pressure. New bearings on a worn journal will quickly seat and be no better than the bearings that you took out.

Best to let them grind it and order the engine kit for you with the correct UNDERSIZE bearings.
Have them check for runout in the big end of the rods to determine if they need truing, also.

Just out of curiosity, did the gasket fail between 3 & 4?

Yes it was between 3-4. I started at 4...judging by where it wore a groove in the block and head.

Have you seen that before?

That's typical of the four-banger. It disguises itself as a cooling system problem because it aerates the system. It is difficult to detect and diagnose.

Be sure to follow the "magic sequence" when torquing the head back down. Go by the book on it. Also go by the book on the timing chain installation. Haynes actually covers that part rather well, but you have to pay close attention.

I checked my service manual...it never occurred to me that I have the specs. Specs for the crank are, based on what he said, as they didn't write them down. (the crank was on the bench, w/the book open in front of him).
rods 1.9670-1.9675 it should have mic-ed 1.9674
mains 2.1631-2.1636 it should have mic=ed 2.1632

The surface on the cranks was fine, no ridges and or etc.

Thanks, for the info. I'm leaning towards not getting it ground. But, I have my reservations about it, for the reasons you suggested.

Unless you have bad bearings that hurt the crank or if you get new bearings that are a little out of tol. don't touch the crank.

The important spec to check is after the bearings are installed in the rods and installed in the mains. Get that reading and then check that against the rod and main journals to find your clearance.

Unlike cylinder rings the rod and main bearing don't ride on the journal surface, there is a film of oil between them so therefore they don't "seat". JIM

Trust me; they'll "seat" if there is .004" clearance with new bearings and uneven journal surface like an old Ford that I did one time.
In other words, if the crank is worn beyond limits, don't wast time and money throwing new bearings at it without grinding.

It sounds like your crank is in pretty good shape, though, Stinky. Still, if the cost is reasonable, I would go for fresh, factory tolerances.

Rod and main bearing DON'T seat. If you have .004 clearance on the rods it's not called "seating" it's called you're going to beat the bearings out of it.

I'm not sure about a nissan but most imports have "torque to yeild" headbolts. You'll want to do some research and see if this is true for yours, if so you'll want to use new headbolts. JIM

If I say "Have a seat." it does not mean to literally take posession of the seat.

Obviously not the best choice of words for you to digest. Sorry to upset you. I meant no harm. let's shake hands and move on with our lives.

I'm not upset, but nobody that builds motors or knows what they are talking about will say that rod and main bearings "seat" to their surface. JIM

Pardon me for existing in your world, but there were no others available.

What's your problem?

You start off by telling someone that has a head gasket problem to have their crank ground without knowing if it is out of spec or if it's been damaged. This advice is wrong. You have to check it first instead of just having it ground down for the fun of it. You also have to check the new bearings in the rods and mains to see if and how much is needed.

Then you go on to talk about rod bearings seating in. This is also wrong.

My advice is coming from someone (myself) that builds motors.

Stinky-- the best thing you can do is ask your machine shop what they think since I don't have it in my hands to inspect it. From what you've said I would think the crank would be just fine. JIM

Stinky, what I meant by the new bearings "seating" on a worn journal is that they can quickly conform to irregularities worn into the journals. A micrometer cannot always measure the depth of tiny grooves that sometimes develop in journal surfaces. Plasti-gage is the best way to judge a proper fit.

That is why I reccommend letting the engine builder/machinist order the bearings. That gives him full control over the final clearances.

My understanding, from your first post, was that the mains are nearing the low end of tolerance. I made the assumption that you intend to keep the truck, so I advised being thorough.

Please understand that my machinist charges me about $45 to grind a four-cylinder crank. I spend more than that on two tankfuls of gas.
If it were mine, I would grind it. That is my opinion. I won't think any less of you if you decide to use it as-is. I'm sure it will last quite a while like that.

I hope that clears things up.
I apollogize to you and anyone else reading this if I acted like an in any way.

Please stop. "Plasti-gage (sp. gauge) is the best way to judge a proper fit" are you kidding me. If your machinist charges you 45/hr and then uses Plasti-gauge to check your clearances, well I now understand.

The bearing DOSEN'T ride on the crank journal this is why they don't seat and they won't develop the tiny gooves. Those tiny gooves on the bearings are usually caused by dirt in the oil. There is a thin film of oil between them, unless you have a deep groove (one you can easily feel with your fingernail) it's when you have your clearances too tight that you have over heating problems and break down the oil faster than normal or have them too loose that you don't get enough oil pressure and beat the bearing out of it.

Go visit a race engine shop and see if there is even any Plasti-gauge in the building. The tolerences on that stuff is worst than someone that doesn't know how to use a mic. JIM

Check the spelling. I was wrong about the hyphen.