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Stock pistons used in a overbore?

Bad B-rad

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12:49 AM
Jun 19, 2020
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I have NEVER done this, but reading some old vintage repair manuals, I have, they say you can use stock pistons on an engine with ten thousands over bore, any larger and you need new pistons.
Is this true ? And would you have to have ten over rings to make this work, or some custom rings?

Once again, I have never attempted this, and I am not claiming it will or wont work, I only read about it, and was wondering if its true on current engines, or just what people used to do in the 30's 40's or a racers trick??
Pistons were knurled to expand them for use in worn bores. I think a lot of these methods were used due to the great recession and 2 world wars and lack of resources.
The repair books I found it in was from early 50's, so you could be correct.
The real question is why would anyone?
I would guess due to the cost involved.
I was thinking maybe back in low HP, low revving engine days, but IDK?
They had no freeways. No high speed driving for several hours like today.
They had no money for new parts or new cars. They used recycled oil.
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Lots of old farm tractors and 30s,40s , 50s cars and trucks got brass shim stock cure bearing knock.
many overhauls just got rings and bearings, no hone job even, just a little comet or bonami cleaning powder through the intake to seat the rings.
My Dad told me about the used oil for his old ford coupe, strain it through a kitchen strainer or simular to get the chunks out.
WOW, running threw a strainer to get out chunks, that's funny.

My grand pop told me that the old farmers used to dump used engine oil on the drive way/ road to keep the dust down!!!???

Looking into it more I found this on a BMW forum :

"no swapping pistons is not necessary. I talked to Jason at pelican parts today and he said anything up to a third overbore (.030") is fine with the stock pistons. He said I should be fine with just +.25mm overbore and rings to match. "-BMW post

I wonder if the parts guy is trying to sell more parts:thumbsup:

Then this on a z-28 forum:
"You could run stock pistons on a .010 over bored engine, I've ran a Briggs & Stratton lawnmower like that for years, it just rattles a little,"

I think that one is funny, it rattles a little, AH really, no crap, its called piston slap!
There is a difference in a one cylinder low HP B&S engine and a v8, with 8 times the rattle, lol!

I found a bunch of stuff about the knurled or knurling the pistons to do this, but all in older engines.
You could do them in the car without having to machine the block. Knurling was more popular than most people think.
The only input I have for this topic is that of my maxwedge. When I first took it apart I found some of the heaviest pistons I had ever seen. They were domed pistons with full round skirts, unknown brand replacements, .060 over. The bores were very nice, but those pistons had .014 piston to wall. I replaced them with standard 440 pistons, arias 12 1/2s, theoretically .070 over, and the piston to wall was perfect.
I built a 383 some years back when cash was tight. I honed the stock bore to clean it up for a true round no taper cylinder and it was about .004 to .005 oversize. So I had the stock pistons knurled and bought .005 over rings. Well the ring gaps were all around .022 without me touching them. The pistons would not go in the cyl after they were knurled so I file fit them to the cylinders for a .0015 to .002 piston to bore clearance and they all fit great when done. The knurl also holds some oil in them which helps keep the cyl wall lubed some. I threw a MP .484 cam in the car with stock rockers and 452 heads I pocket ported. Ran an RPM intake and 750 DP. I ran that eng for 6 years and it never smoked and never burnt any oil. And in a 72 Dart with 3.91 gears weighing over 3600 lbs with me in it the car ran 12.31 @ 110 best. I never had a bit of trouble with the eng and after 6 years I sold the car to my son real cheap. Ron
For when those rod caps have a little to much play.
Are those sandwiched in between the (back side) of bearing and cap?
I have even heard of head gaskets being reused in the 30s and 40s on farm tractors. Money was extremely tight and many parts were not available during the war years.
I have even heard of head gaskets being reused in the 30s and 40s on farm tractors. Money was extremely tight and many parts were not available during the war years.

Yes, My grandad only had straight water in his tractor and had to drain it each night in the winter if used during the day.
Years back my son was into demo-derby. Radiators were a problem so he wanted to run the whole derby without water......start to finish. Enter our waterless engine with stock pistons and .013" piston to cylinder wall clearance. Sometimes the car would fail but never the engine. We did find that the cam bearings would melt right out of the block in big drips.

After one derby there was a meeting to decide if they were going to ban Chrysler Imperials. Instead they decided to ban our waterless engine. I guess they figured if we started with water and then ran out of water the engine would quit?????
Demo derby approaches are really interesting. I observed that Mopars did real well, no shift linkage, just bolt or rod up through the floor. Wagons did the best. lasted a long time with the water gone, the unibody could bend way up out of shape & keep going as long as the angle of the drive shaft didn't get too far out.
I did Demo Derby for 20 years. No radiator, rod for shifter. I built my own steering column, slip driveshaft with Imperial CV ends. A sloppy 383 I ran in 67 heats till broke mounting ears of block
To OP, I recall a couple of core motors I got in the 70's & 80's with knurled pistons. Mopar blocks don't wear so much so the knurling works well.