Do I need to bore over 30 to put in pistons?
 

I'm thinking of putting in some flat top pistons in my inline 250.

I feel the engine is fine , but I was told I would need to have it bored if I was going to go with new pistons.

I really don't want to bore the block.

So will it be OK , to put in new ones without boring ?


Zac

Re: Do I need to bore over 30 to put in pistons?
 

If you want oversize pistons, you must have it bored.

Depending on how much of a ridge you have at the top of your cylinders, you may get by with just a hone & new rings on your existing pistons.

You are best off going to the machine shop.

Re: Do I need to bore over 30 to put in pistons?
 

it would be best to bore it. chances are you have cylinder ware.. but you could get away with it if you use the same bore pison thats in there now.

Re: Do I need to bore over 30 to put in pistons?
 

pull it apart and take it to a machine shop they will tell you if you need to bored it or not but if the motor is old you will probley need to bore it i am building a 305 that i had bored 30 over it doesn't hurt the motor any it helps with performance

Re: Do I need to bore over 30 to put in pistons?
 

It all depends on the amount of cylinder wear. Is there a lip at the top of the cylinder? You should have the bore checked by a machine shop, they will be able to tell you in minutes whether or not you can replace with standard size pistons or if it will need bored. Why the change to flattops? Are the ones in there now dished?

Re: Do I need to bore over 30 to put in pistons?
 

Wow, 4 like answers inside of 2 minutes! Guess that should confirm the answer to his question!

Re: Do I need to bore over 30 to put in pistons?
 

I was told they were dished (1970 -250). I don't know though.

I'm planning on pulling it and painting / headers/ 4bbl intake.

So I thought I might dig a little deeper while I had it out,

Re: Do I need to bore over 30 to put in pistons?
 

I say, if you're gonna have it out and torn down and you're replacing the pistons anyway, have it bored .030 over. I bet those pistons would be easier to find than stock...

Re: Do I need to bore over 30 to put in pistons?
 

Might as well throw a higher-lift cam in there while you're at it.

Re: Do I need to bore over 30 to put in pistons?
 

anytime i have a block bored i go for the max// cost the same 30 or 60 pistons cost the same rings cost the same ITS FREE CUBIC INCHES

Re: Do I need to bore over 30 to put in pistons?
 

you can use new pistons in the standard bore but you'll need to have someone measure your cylinders to see how much it's worn if it's excessive you'll need to bore it. me personally i don't like to bore the block more than it needs because if you have problems later you may need to bore it again

Re: Do I need to bore over 30 to put in pistons?
 

I usually agree with cdowns on most replies but, not this one.

The main reason for an over bore is to provide a new and proper cylinder bore. Not to add HP by adding cubic inces.

The in crease in HP out put by boreing in very very minamal on the cubic in" matter. The real gain is from attaining a new and proper piston to cylinder enviorment thus increasing HP output.BETTER PISTON AND RING SEAL

IMO every thing that was stated about having the machine shop check the block out is right on the money and if the block does need bored go with the minimum over bore allowed to take out any imperfections and out of roundness.

Since this is on a 250 there is probably not as many options out there as say a 350 sbc but its worth checking into IMO.

Re: Do I need to bore over 30 to put in pistons?
 

Speaking about boring engines. Sm block 350's for example. Some folks are going 60 over. Doesn't that cut pretty close to water jackets? Until recently I had never heard of going 60 over. Now several shops do a 60 over bore. Old school was always stop at 30 over( 283/327's).

Re: Do I need to bore over 30 to put in pistons?
 

In general, going .060 over will decrease the cylinder thickness and could possibly increase water temps, but usually not noticable in most small blocks. However, 400's are more prone to that problem when going over .030.

Re: Do I need to bore over 30 to put in pistons?
 

Core shift comes into play on some blocks and I always recommend that a sonic test be done when boring upto and including .060 over.

I agree with what glock35ipsc said on the matter.

Re: Do I need to bore over 30 to put in pistons?
 

I think if there is any significant amount of mileage on that block (50k+) then you're looking at machine work. And why not? Why would you want to tear down the motor and up the compression, only to risk having it smoke like a tire fire when the rings don't seat correctly?

Another thing to think about is future work. If you go .060 over now, you're buying another block if something happens to a cylinder wall.

Re: Do I need to bore over 30 to put in pistons?
 

Take it to the machine shop. I bored my 250 .040 and have been extremely pleased with the results. IMO, the 250's are work horses that were normally worked really hard. If you're going to take the time to rebuild it do it once and do it right.

Re: Do I need to bore over 30 to put in pistons?
 

:agree:

Re: Do I need to bore over 30 to put in pistons?
 

i agree with whats been said and i also very much agree with NOT going .060 over right now if .020 over will clean it up then do that if it takes .030 then stop there, dont bore out anymore than you need to clean it up, if you want more power out of it rebuild the head with valves and springs and add a cam and stroke the block a bit and theres your HP

Re: Do I need to bore over 30 to put in pistons?
 

A 250 L6 @ stock stroke bored .030 over has 254 cubic inches.

Re: Do I need to bore over 30 to put in pistons?
 

the only reason i bored my 305 30 over is because in case it ever gave up the ghost i will be able to bore again

Re: Do I need to bore over 30 to put in pistons?
 

thats the exact reason why you shouldnt go the max on the first time out, never know if youll have to bore it again

Re: Do I need to bore over 30 to put in pistons?
 

Have 'em throw on the torque plates while your at it, makes for nice straight cylinders.

Re: Do I need to bore over 30 to put in pistons?
 

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I agree with Sammy!!! I would think more then 40 over on a 350 is the begining of leaving the comfort zone. Some old 283s were bored as much as 80 over but I would not go more then 60 over if it were mine. 327 same thing 60 over is fine but a 350 40 over max.

Where is longhorn Man when you need him.

esll. :)

Re: Do I need to bore over 30 to put in pistons?
 

Typically blocks are cheaper than good pistons. So if you bore to the max and something fails, you'll have to replace the block and rebore. If something goes wrong and you damage a piston, you'll probably not have a block either.

There are a lot of theories out there. . Start with the machine shop, decide from their.