I was wondering if anybody here has bored a 300 .07 over. the block I have is maybe a .06 over the shop doesnt really no yet. If not I was thinking about just useing .02 over 390 piston. The truck doesnt get driven alot maybe 2500 miles a year at the most. any useful information would help. Thank you

 

40 over is safe. 60 over is borderline but doable. Any more than that, probably best to have it sleeved if the block is otherwise usable.

 

X2 what Harte 3 said.

Also, it is always a good idea to set a goal with your engine before you simply start slapping parts together. If you go with a 390 piston, which I have heard of doing with .050 over bore, how much will that bump the compression? Do you want to stay on regular gas? How many hp are you shooting for? Do you want a radical race engine, or one with quite a bit more hp than stock, but still great for long trips and as a daily driver?

Boring a block is done to remove scratches, etc. in the bore. Does it need to be bored? Would it be possible to just hone it? Ask your machinist.

BEFORE YOU PROCEED with any machine work, have the block magnafluxed. A little insurance goes a long way.$

It's just me, but I like to have as much 'meat' in my block as possible. So I don't build over .030.

 

Why punch it 30 when 20 will do? Everyone goes 30 never knew why. Heck I would go 10if they made pistons.

 

Hey flip--go to the 73--79 F series forum, and check out my thread entitled, 'disc brake conversion.' There is great conversion available for like $270. A guy posted a link to photos, etc.

 

i think punching .030 over came from the v8 guys, i guess .030 is so popular because the piston bore gets oval on the bent 8, and thats the only way to get them back into perfect circle, i dunno

 

pretty sure they do...check on summit


390 pistons from what i have heard create too much compression to continue runnin pump gas...


.060 over is max without sleeving it...but over boring a 300 wont get you much if any thing...why sacrifice the reliabilty for tiny numbers? ... bore it only enough to clean them up and get hypertuectic dished pistons and deck the block and head to create the compression you want...

 

Well said. Only bore it over just enough to clean it up. Overboring just to be able to brag you have a 305 I6 isn't worth it.

 

The reason I was asking was because one cylinder has a huge scratch done it and the machinist doesnt no if he can get it out at 60 over so I was wondering about 70 over and 390 piston 20 over would be the easiest way. thank you for all the information. I will just have to wait to see what he says.

 

If I were you, I'd pull a short block from the boneyard for about $150 and take THAT block to the machine shop before I'd put all that time and money into trash block. "Why is there never enough $/time to do a job right, but always enough to do it over again?" Find yourself a stock bore block, and go from there. Anyone agree? Pull the head first, before pulling out an engine at the junkyard, and check the top of the pistons. They will say: .020, .030, .040, .060. If they don't one of those #'s on top, then it is a stock engine.

 

X2 ... if the scratch in the block is that bad it isnt worth fixing unless you really wanna sleeve it and by the time you do that it would be cheaper to get a block out of the bone yard...and X2 again on diggin into the engine before you rip ir out, its a lot of work but its better than ripping the whole thing out and paying for it to find out its no better, or even worse than the other one...when browsing through the yard look for truck that already have much of the top half ripped appart, even one that the intake is removed makes that much less work

then you can sell the old block for scrap and make some money back...if i remember correclty the block and pistons weigh in around 390lbs? i think...that should make some coin

i like that quote haha...its very true