Help...Little shop of bores needs Pyschic

[Matthew A.]

My engine needs a bore job. I am uncertain how much over-bore the cylinders are going to require. The machine shop needs the new pistons inorder to match cylinder bore with that of the piston. Yet, since I do not know how much the cylinders will have to be bored out too, I don't know what size pistons I will need to order...
But...I can't get the cylinders bored until I order the pistons.
I can't order the pistons until I know how much over the cylinder bore is going to be... I don't know how much over the cylinder is going to be cause the machine shop can't bore the cylinders until they have the pistons...

Oooh...I'm getting dizzy

 

[baxtr]

Re: Help...Little shop of bores needs Pyschic

Is this a machine shop that regularily does outboards? I know the repair manual says to have the pistons at the machine shop so they can bore it to tolerances, but when I took mine in to be bored, I asked the same question, they asked which pistons I was buying, they then knew what sizes were available, and bored it to the lowest overbore that got rid of the scoring. I didn't need the pistons in hand at the machine shop. I understand your dilema, what happens if you buy .020 over and they have to go .030? then you have a set of brand new pistons for sale. Can they tell you how much they have to oversize your cylinder before you buy them? Just make sure they do outboards, and are not just a automotive machine shop.

 

[roscoe]

Re: Help...Little shop of bores needs Pyschic

The machine shop should be able to measure the bore and determine to what degree it needs to be bored.

 

[Frank Acampora]

Re: Help...Little shop of bores needs Pyschic

Nah! Nah! Nah! Matt --go to another machine shop. This guy doesn't know what he is doing and is making your life unnecessarily complicated.

First you mike the bores to see how much overbore they need to clean up: .020, or .030. (WISECO shows an .040 over piston, but Chrysler manuals state to not bore more than .030.) So, only go .040 if it means that otherwise you would need to throw away the block.

Your Engine has a stock bore of 3.312 inch. So the machinist bores, rough hones, and then finish hones to either 3.332 or 3.342. The pistons are machined to properly fit these exact bores: ie. provide the proper wall clearance. If your machinist doesn't know how to leave himself two to three thousanths to rough and finish hone, he shouldn't be working on engines.

If by some chance you have a block with the 3.375 bore, they sell pistons .010 over so if the damage is really minor, you could get away with this minimum.

At any rate, only overbore the smallest standard amount necessary to clean up the damage in order to preserve as much cylinder wall thickness as possible..

 

[steelespike]

Re: Help...Little shop of bores needs Pyschic

Not an expert but I believe I've read here that you must take it to a shop that regularly does out boards.In my opinion you take the block to them they should be able to find out what pistons are available and bore it to the size.No way to know what to bore without knowing what is available.No way to know if it may allready have been bored before or how bad it is without checking it.You could find out what is available yourself and they can make the decision if it can be bored and to what size
based on what they measure.

 

[WillyBWright]

Re: Help...Little shop of bores needs Pyschic

Two factors that make outboards radically different from car motors are that the pistons are made slightly oval and they need a lot more clearance. The blocks cannot be accurately bored based on the piston because of it's slight out-of-round geometry. They must be bored based on specification alone. The shop is capable of doing it. They just might not know about this odd quirk.

The factory manuals have excellent diagrams with measurements (specifically Standard Bore). I just photocopy the appropriate page and state .010 or .030 over.

 

[Scaaty]

Re: Help...Little shop of bores needs Pyschic

Whats a stock bore size from the manual? Whats the next size over bore piston? .015? Pretty simple to add .015 to the stock bore size and do it..