advice on bore and pistons?

[saridout]

i've miced my cylinders, and the bore on the whole block ranges from 3.3855" to 3.3895", with seemingly no pattern as to which diameter is where. i've never done this before, so please forgive me for asking: is this enough to determine whether i can use standard pistons or not? i'm going to scan the full list of readings and post it soon if more info is necessary.

i've been finding some conflicting info about the L28 standard bore. my fsm says that standard is up to 86.050mm, but a piston source i'm looking to buy from says that their standard bore piston is 86.1mm in diameter.

lastly, does anyone have any horror stories about black dragon pistons or ITM pistons, or should i not have any problem with either?

edit: also, new photos in my flickr set. enjoy!

Edited July 21, 2009 by saridout

[ChrisZ]

All you need to know is in the FSM (factory service manual) search and you will find it, lot's of threads about it on this site.

Chris

[saridout]

i've been searching, actually, and i can't find what i'm looking for. if you search for "280z standard bore" only two results come up, and neither one are useful to me, etc.

[ChrisZ]

Search after FSM

[saridout]

i'm sorry, i don't understand :/

[bigoak]

FSM=factory service manual...

You can download it. Info will be in there....

[saridout]

that's not the part that i misunderstood. as i said above, i've looked in my fsm, and i'm trying to find out why some retailers say that 86.1 diameter is standard, when the fsm says that maximum standard is 86.050. the piston brand i'm looking at is ITM. i'm sorry if this seems like a stupid question, but i've never rebuilt an engine before and i don't know any better.

[Phred]

"i've miced my cylinders, and the bore on the whole block ranges from 3.3855" to 3.3895",

Don't fret over what a std. bore is supposed to be. Your real problem is in your quote above. You have a .004 variance in bore size. If you stuff a std piston in there, you will have too much pist./cyl clr. I don't know what the FSM says, but new stock pistons in a fresh bore should only have around .0015 clr. And as a general rule, most books call out 86mm as std, which is 3.3858. As a machinist/engine builder, if I came across a block like this, I would recommend that it be overbored to the first O.S. piston that was available. Generally, that would be +.5 (.020) or a 1mm (.040) overbore. The other problem in putting a new std. piston and new rings in an oversize/out of round used bore, is the difficulty in getting the new rings to seal up properly. As you can guess, a new ring will have major problems following a bore that is not straight or round. The usual results are an engine that smokes, and generally will not run well as the fuel/air mixture is being contaminated with oil. I could go on and on about this, but you get the idea.

Phred

[saridout]

phred-

just to be sure, your advice would be to get it rebored to +.5 and get a new set of oversize pistons?

back when i was driving the car, i was having a lot of trouble with it idling rough and seeming to run rich. could out-of-round cylinders have contributed to that?

[ezzzzzzz]

There is too little info to understand why you even got to this point. If you need to rebuild the lower end then get to it. Get the machine shop to properly mic the bores and determine how much larger you need to go, either .020 or .040 oversized. Order new pistons and rings. This isn't rocket science as someone once said. As for the original running condition? This and a whole lot of other issues could be the culprit. You need to get the basics in order first. Personally, I think you jumped the gun unless you had a very good reason to pull the head off to check the bores. You should have started with a proper tune-up. There must be Z people, enthusiasts and professionals, in your area that should be knowledgable and willing to help you.

Edited July 22, 2009 by ezzzzzzz

[saridout]

i apologize, i've been posting a lot here about my medley of problems, and basically the same people have been commenting, so i assumed that whoever would be seeing this post would be familiar with my situation. my z overheated and warped the head about 2 months ago. if you would like more background information, which i don't think pertains to the topic at hand, look at these links:

http://www.classiczcars.com/forums/showthread.php?t=34462

http://www.classiczcars.com/forums/showthread.php?t=34581

http://www.classiczcars.com/forums/showthread.php?t=34586

believe me, if this work wasn't absolutely necessary, i wouldn't be doing it. i don't actually enjoy tearing apart my car for kicks. i can't afford to have a professional do it all for me, and up to now my local z club, which is an hour away, hasn't been very helpful.

the machine shop i've been working with told me that it would be better to bring in new pistons along with the block so that it could be bored exactly to the specs of the pistons. so at this point i'm trying to figure out what i need; whether i can use standard size pistons or i need to go up to the first oversize.

i haven't just taken my head off, i've pulled the block and removed all the pistons, which were pretty scuffed up. the cylinders are scored vertically, and 4 of the pistons had broken rings. how reliable are datsun connecting rods? are they ever known to break?

[64imperial]

Maybe you've misunderstood your machine shop, they should bore the engine and THEY will tell YOU what size pistons to order. Then they will finish up the machine work when the pistons have arrived. Say you mic your own cylinders and believe a .010 over piston is adequate. BUT the machine shop needs to take it to .020 over because of scoring on the cylinder walls.

You can't possibly pre-determine or even see minute scoring. You can't know the piston size you need until the machine shop begins the process.

Let the pros do their job with their proper tools (internal bore mics, etc) and if they've misadvised you, go to a better machine shop. Spend more money now at a quality machine shop rather than doing it twice for 3 or 4 times as much after correcting poor work and ordering wrong parts.