Technical Olds flat 6 Piston size dilemma?

Alright so lemme take it from the top;


i picked up an olds flat 6 for my coupe im building.. it came with a buncha parts in the back of the shed of which they were all stored together.. Initiaially.. I wasnt keen, but thought what the heck and took all the parts.

Get the motor and parts home and pull the head. The bores been honed out to 3 1/2 with 3 7/16 pistons. Nice. So someone honed out the 230 olds to a 238.

It was all locked up and the sump was rotten etc so i picked up a parts flat 6 for the sump etc, but moving on im sorting new pistons for the motor as one of the ring lands broke when i was trying to get the stuck rings out - no one to blame but myself!

So here's where it gets tricky.. I picked up a set of 238 pistons which turned out to be 30 over and measure at 3 n a half. So logically, id need a set of standard 238 pistons which would make 3 7/16 to fit into the 3 1/2 bore, am i right?

Im relatively new into getting right into the guts of a full motor tear down so originally i thought the bore means the piston size you buy - didnt realise the piston is actually smaller than the bore.

Now continuing on with where i said it was tricky - all the 238 pistons i can find, they say the piston is 3 1/2.. are they being a goose like me thinking the bore is 3 1/2 so therefore the piston must be, without measuring the piston its self.. or whats going on? The 30 thou over 238 pistons i have here measure at 3 1/2 and the pistons i pulled from the bored out 230 to make a 238 are 3 7/16.

Reason im apprehensive is cause postage costs ALOT for anything to Australia and i dont want to buy another set of pistons which dont fit the motor.. Hell, At this rate if anyone has any leads on a set of standard 238 pistons measuring at 3 7/16 theyre welcome to have the 30 thou over pistons sitting on my desk..

Hope someone can chime in here on what the go is.. Cheers
Jackson

 

How are you determining the bore size?

The best thing to do is measure the bores with a bore gauge or inside micrometer. Piston to wall clearance (difference between bore diameter and piston diameter) should only be a couple thousandths at most. For example, my 1955 Oldsmobile service manual says .0005 (half a thou) to .001 (1 thou). Yours is probably not the same- especially if you have cast iron pistons. Never been in an old flat six, so I genuinely have no idea. I only dabble in building engines as needed so the true engine builders will inevitably chime in with more concrete figures/recommendations.

Your 1/16” indicates a roughly .060 overbore. .030 over pistons are way too small if those numbers are correct. Measure everything precisely to .001” and determine what you need, or if the bores are even useable as is. No use putting new pistons and rings in bores that have a nasty ridge or aren’t even round anymore.

 

Man what a coincidence. I’m working on an olds 303 motor that came complete except for the piston rings. The engine had been bored from 3.75 to 3.90 with what appears to be Chevy ls pistons installed except the oil ring width is off. The hunt for rings that fit was a challenge but eventually figured out.
Working on these old engines can be really confusing and challenging.

 

I forgot to add - the motor has been rebuilt and sat in the back of a shed on a dirt floor. It has Australian pistons in it which are oversized to a degree.

An olds 230 which is what the was originally should have a bore of 3 7/16 where as this has the bore of 3 1/2 with pistons which were 3 7/16 in diameter I pulled out.

On the note of that the parts motor which is buggered has a bore of 3 7/16 and the piston is significantly smaller.. I’d have to remeasure. Definitely oversized so hoping since a 238 has a bore of 3 1/2 the standard pistons to fit that should be what I need.. just need to confirm if those pistons are 3 7/16 in diameter since all the eBay sellers are saying they’re 3 1/2 which can’t be right.. I’m sure they’re just going by bore size!!

 

Your going to want 3 1/2” pistons for a 3 1/2” bore not 3 7/16” because like oldsmobum said a 1/16” difference is too much.
you really do need to mic it out before making your purchase.

 

How do you know? How did you measure it? Tape measure is meaningless. You have to ditch the fractions… Once you have a motor apart, it is no longer accurate enough to reference the bores/pistons dimensionally in fractions. They are somewhere between 3.4375 and 3.500 (maybe). Every .001 matters and needs to be accounted for before ordering parts or you’d better not skip leg day at the gym because you’re going to be pushing the car around.

 

I've since come to that realisation.. ha
I measured with a tape measure, you got me!
Anyway, moving on though the issue is the motor has been bored out to some size other than stock since i have a 230 parts motor also with a significantly smaller bore.. This one also had nice cross hatching and made in australia Rolloy Pistons - Part number S3207.. Which leads me nowhere. So with the pistons ive already got, where should i take em to measure what size is what i need? A decent micrometer locally is the better part of the price of another set of pistons.. Australia sucks for buying tools at times. I suppose at a guess a rod shop of sorts may have the tooling.. Drawing a blank here.

 

Any machine shop would be able to measure them. Even a good mechanic should have the capability of measuring them, and might do it for beer. Alternatively, you could look into buying a bore gauge set… they range from dirt cheap (questionable) to expensive like most other things… I have no idea what they cost in Australia though. A set of digital calipers will be accurate enough to tell you the size of the piston, and they also have a big range. Swap meets and garage sales can be a good place to buy tools as well.

Unfortunately unless you have friends with tools, this is going to cost you one way or the other… But if you’re careful it doesn’t have to bankrupt you.

 

Thankfully i have a few friends who can lend a hand! It'll be what it'll be i suppose.. we'll get there! Cheers

 

I'm no pro, but pretty good at all this stuff. I can measure everything myself, got some pretty good micrometers. But not cylinder bores, I get my machine shop man to do that if I want to know exactly...........like if buying pistons and rings.