To bore or just hone?

[cajunz]

I've got the engine from my '78 pulled and dissassembled (N47/N42). The crank, cam, and rods look to be in good shape. Probably just a polish on the journals. The bores have a very faint ridge at the top and there is some scuffing on the piston skirts and bottom of each cylinder bore. I checked the bore taper on each cylinder and came up with .0018 taper across all cylinders. My question is, do I need to have the block bored/honed or am I good with just honing? There aren't any mechanics/machinists around here who work on these engines. The one machinist that I talked to said he recommended boring out .020 over. He said he could do the the bore/honing, checking the head/block mating surfaces for trueness and machine if neccessary, and rebuiild the head for between $500 and $700. This is my first rebuild and I want to make sure I do it right. Any advice from the experts would be greatly appreciated.

Brent

[beermanpete]

The factory service manual specifies the maximum cylinder taper at 0.0006" (0.015 mm) taper. Your taper is well beyond the max allowed and is also equal to the maximum specified piston-to-cylinder clearance. Boring the cylinders seems to be required.

[5thhorsemann]

Thats not a bad price for the work either.

[Lazeum]

I would redo the engine. It will allow you to start with new pistons/rings also.

Do it right now, you'll be glad in the long range & you won't have any second thoughts once everything will be over.

This is the route I've taken for my engine

[cygnusx1]

Yup bore it. It will be good for another 200k miles. Make sure you involve the machinist with the pistons and rings before he does the boring. He needs to check all the clearances specific to the supplied parts.

rpmrons.com has tons of valuable info that is interesting and informative. The layout of the site is a bit rough, but it's worth the dig.

[cajunz]

Thanks everyone for the advice. I think I'll just go on and have it done right. Now to source the parts. Cygnusx1, I'll check out rpmron's and see what he has to offer. I'll keep you guys updated on my progress.

Brent

[Z Greek]

How did you measure the cylinder taper? An accurate dial bore gauge is required to accurately measure this, and it takes practice to get it right. I suspect the ridge you feel is just carbon. How badly are the piston skirts scuffed? Do you have any pictures of the pistons? Have the bore measured by someone who knows what they are doing, and make your decision from there. I am all for "doing it right," however, if you can save a few hundred on boring, and new pistons, and still end up with a quality job, the money can be spent elsewhere, (like on the head, that's where the power is) Datsun blocks are VERY durable, and the original parts are very good. Also, I would highly recommend accurately measuring bearing clearances with replacement bearing shells, I have seen new standard aftermarket bearings with too much clearance on a polished crank which is within tolerance That's my two cents.

[cajunz]

I measured the bore taper using a piston ring and measuring the gap with a feeler guage. Took a measurement just below the ridge and at the bottom of the bore. It was one option mentioned by Tom Monroe in How to Rebuild your Datsun/Nissan OHC Engine. He said that the max difference in the gap measurement for honing was at .006, which was what all my bores were at. Why would you think that the ridge is just carbon? The engine has approx 187k on the odometer. Here's a pic of one of the pistons. It's pretty representative of the six.

[cygnusx1]

Piston skirt scuffs are not indicators of problems at all. That's normal. Even low mile pistons can look like that. You can probably throw in new rings, give it a hand hone, and be OK for another 200k but while it's apart...nuff said.

[Z Greek]

I say the ridge is probably just carbon, because in these engines, it usually is! These blocks are very durable. I agree with cygnusx1 above, except I would invest the money and have a machine shop hone the block on a Sunnen machine. Properly done, the machine will take out most of what taper you have (depending on what it actually is), hand honing will only add to it. I qualify all this by saying only make this decision after you have the bore accurately measured at a competent machine shop. Since the block is already there, then you can make an informed decision whether to have it bored or not. You can order pistions and rings at that point if necessary, and have them there in 2-3 days.

This is my opinion, and you know what they say about opinions! Good luck!

[cajunz]

Good advice Z Greek. I'll have it accurately measured to see what I actually have. Hopefully I can find someone with the Sunnen machine. I was looking in the bores with a good light and can actually see some of the crosshatch markings are still there.

[cygnusx1]

Here you see about 150k miles with about 30k of those miles running 16psi boost making 300+ wheel horsepower, with stock long block. Bores still have the factory hatch. I was changing the head gasket. Later I turned up the wick to 18psi and eventually shatterd three piston skirts. The bores still looked fine. I bored it anyhow. In my book, with new pistons and rings, with over 150k miles, merits a rebore.

Ultimately as the guys said, it's about the measurments, not what it looks like. Interestingly, even though my bores were slightly out of spec in taper and ovality, you could still see the crosshatch all over. What does that mean?

A) either the factory never made perfect bores in the first place.

or the block deforms with age.

c) or true measurements were not obtained because I didn't use a torque plate to mimic the force from the head bolts.

D) all of the above.

Edited March 9, 2012 by cygnusx1