70 block bore condition

[Dennis D.]

Looking for opinions on the bore condition before my L-46 block goes to the shop. The block is very clean and it seems to me the crosshatch pattern from the last rebuild is still there. Is that a good thing? Did the rings not seat? Lot of crud on the piston tops and in combustion chamber. Think this is from a manifold miss-alignment,and not ring blow-by, since the head intake runers show signs of oil. Also, the backs of the intake valves are crudded up just like the combustion chamber.

I checked bore runout the crude way, with the piston ring gag. There is literally no tapering in 3 locations and at 90 degrees. Wear checks out roughly .001 to.002 from the ridge to midway down. Pistons come out with no ridge resistence. Piston clearence seems a little large, (given the other checks), at .004-.005.(feeler gauge at skirt).

I tore the engine down when I found the bearings trashed.(earlier post) I hope the bottom ends checks out to be fine, because the upper end is another story. Will post for help on that soon.

[Dick W.]

Re: 70 block bore condition

Dennis, appearance wise the bore in the photo looks good. I believe that the cross hatch of the honing is magnified by the camera. I would not be comfortable with .004-.005 clearance on the street. Piston slap and oil control could be a problem. The taper, .001-.002 is acceptable. I would check the valve stem clearance and correct if necessary and use positive valve stem seals. I think that it is your call on the bore clearance. If you are going to leave the bore alone, I would run a "burr" lightly in the cylinders to break any possible glaze. I would then have the engine "tanked" to clean it. Bring it home, wash the cylinder block with soapy water and rags, then take clean rags and motor oil and clean all the bores, cylinder, bearings, tappet, continuing until no trace of dirt is on a clean rag.

[Dennis D.]

Re: 70 block bore condition

Thanks Dick. I was debating having the shop that tanks the block leave the freeze plugs out, so I can clean it out at home. The last time the shop installed the plugs, so I wasn't able to wash out all the bores.

With the .004-.005 clearance, should the crosshatch pattern still be there with that kind of wear? I would think it would be gone, unless the shop that did the block before went to far with the bore/ honing. BTW, the piston/ skirts show very little wear.

One other thing I noticed on dissassembly. When I pulled the main bearing caps the oil soaked bolts had a distinct oil-smokey odor. Any thoughts on this being related to a ring sealing/ clearance issue?

[Mike McKown]

Re: 70 block bore condition

I believe if that was my engine, I'd put the top end back together as it is assuming you had no piston slap on a cold start before you took it apart. I can't tell but if those are forged pistons you have in there, 4-5 clearance is probably ok. Your machine shop should know.

If you lost the bearings be sure and pull not only the welch plugs but also the threaded plugs in the cam oil gallery.

[Dennis D.]

Re: forged pistons *NM*

[Dick W.]

Re: forged pistons

I have always tried to maintain about .001 skirt clearance to 1" of bore for a street engine. For some of the real screamers I would use .002 or so clearance, but oil control can be a real problem much past that. As Mike stated, if you had no piston slap before you should be able to reassemble you engine with no problemsYour engine was put together very clean judging by the piston skirts. No trash in the bores when assembled. Apparently you have also taken very good care of it. Be sure to ascertain what caused your bearing failure, don't just repair the effects.

When you have the block tanked, remove the oil gallery plugs and use a bristle brush to clean the oil galleries out. Sludge and wear material can hide there and come back to bite you at a later date.