i would like to bore my engine oversize but i got people telling me different things and just want to see what you guys have done.

if i bore it over and cause these engines are work horses will boring it over even do anything or just make me suck more gas?

what have you guys bored your engines over to and how much extra power have you seen? i know these engines wont see good power increases but just even a little bit extra will make me happy.

i have headman headers on my truck now and have seen a little increase in power and pulling power but im thinking i might go back to stock efi heads cause then i can have the egr hooked up and help me get better mpg. witht he headers i get 12.5 city and highway just in town but when i went mudding on laborday i had to go get parts from another town 40miles away for a friends mud truck and have gotten 16.5 mpg but that is all highway doing bout 60-63mph.. im just wondering if i bore it over if i can get good mpg again or just to stay stock and just get everything checked out. i also am thinking about putting a cam in and possibly the chebby 250 inline6 rockers on mine

 

Just my opinion, but I see no significate gain to boring any engine. Boring is needed only to fix damaged cylinder bores. Cam, porting etc. is where your gains come from. Plus, too much boring causes heating problems and weakened cyl bores.

 

The only times I would consider overboring an engine block are:
- When the cylinder walls of one or more cylinders are so scored that just honing them would not take care of the scoring,
- When the cylinder itself is out of round, and you need to bore it oversize to get it round again,
- When there has been core shift and the cylinder centerline is no longer square with the crankshaft centerline.

These are the only times that I have ever overbored an engine.

The negatives of overboring are that the cylinder walls are thinner, thereby allowing more heat transfer and allowing the cylinder to flex slightly more, and that you will have to buy oversize pistons and rings that (on some engines) are not available unless custom made.

The only positive reason for overboring that I know of comes with some engines (335 series and modular engine series right off the top of my head) that have a canted valve cylinder head where the valves may be shrouded by the cylinder bore. This would be the only time that an overbore would be beneficial, just to unshroud the valves.

As for any performance benefit, I have seen very little, even on the 4.9/300. The cylinder head ports are the major limiting factor in this engine, not the engine block itself.

 

Just to pile on the points already made, standard overbores are not enough to provide enough extra cubes to notice.

A 300 bored .060 over ends up a 309. 9 cubes in an engine with 1/2 hp per cubic inch in stock form is 4.5 hp at 3400 rpm. Think you could feel that?

Not sure you could go much over .060, and if you did, the overheating and cylinder wall strength issues would start to surface.

 

I'm running .080" over right now without any overheating issues. I'm actually running cooler than the old engine since I'm not having to stand on it all the time.
I do agree, though, there isn't a whole lot to be gained by boring it over. If the engine has significant wear, then yes it should be bored.
You could always go .050" over and drop in standard 360 pistons, and have a nice high compression engine

 

There are some EFI guys on the forum who have done some very impressive things with their engines, like porting, cams bigger than I thought possible with the computer, tricked out injection mods, and more.

If I were you, I would post a thread: Who is running EFI mods? or something like that, and start to learn about what is out there and how to do it. If there's a will, there's a way. I hated it when my dad used to say that! But it's usually true.

Cheers.

 

yeah i will check the tapper first. when i got the head redone in april i never leaked any oil now i leak oil more than average. i know i have a ticking noise in the bottom end cause it didnt stop when i replaced the lifters cause i thought one was bad. i might throw a cam in. and i have ported the head when i had it off, i notticed a little more gain.

does anyone here know about useing chebby rockers? or does anyone use them with no problem.

 

The only way you can tell if you need the block bored, is to have the pistons gone and mich the bores.

Ticking is the sound headers make when they start to leak. You need studs with TOP quality crimp nuts, or Stage 8 locks from Summit Racing. Use a Mr. Gasket #260.

Leaking and burning oil are two very diff. things. Burning oil is a sign of a worn engine, ie. shot rings, leaking v. seals, etc. Leaking oil could simply be a seal or a pan gasket, tightening the side cover, or the fuel pump. Don't assume it is something and tear into the engine w/o knowing.

Chevy rockers are IIRC for pre EFI engines/heads. You need the 'slotted' p. rod holes because the the p.rod is moved closer to the rockers. Touch=heat, which could lead to a costly meltdown.

 

i know its not an exhaust leak because it only happens when im sitting in an enclosed area outside and i give it just a tad bit gas and it just ticks or maybe it is a knock. maybe it is a bad lifter yet cause after i put the head on it ran on 2.5 qts of oil for like 2 months before i checked it and the new lifters went bad i have added stuff to the exhuast to quiet it down and i dont hear a thing when i rev it. maybe just the cam will do no rockers.

 

would this comp cam be good
COMP Cams 66-248-4 - COMP Cams High Energy Camshafts - Overview - SummitRacing.com.

how much more valve lift will this give me?

 

Go directly to Comp and ask them. COMP Cams® - Technical Support and Contact Page

 

might be my lucky week. walked into the one engin rebuild shop down the road from my work and as i was waiting for the guy to get off the phone i noticed a straight 6 sitting their and the guy said that someone wants him to sell it for him. it depends on how much he wants for it i might just grab it and use the parts i need and just keep the rest for spare. its just the block, cam, crank shaft, and pistons.

 

well here are the pictures of the damage. the piston is shot cause it broke from the rod hitting it and the corner of the cylinder wall is chipped. i dont know what i want to do. im just going to take and get a new rod and piston and put it back in. i like the straight six cause it has been good to me the past 2 years. i just got a new job that pays good in april and have been thinking about getting a new truck but i dont know the way the econemy is i would like to just save up. would this last long if i just do the piston switch? if i can get til spring with the truck and have more money saved up to put down on a new truck would be good i think, i would go to dealership now but its not worth anything. if i have it running i can use the trade in as a down payment too. i hate to see the straight six go but you have to make sacrifices.
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the other thing i am worried about is if i want to bore it over would that chip in the cylinder be a weak point?

 

The more I look at it, the more it looks like a crack just above the impact point. If that is not a crack, though, just bore it whatever it needs to clean up, and ignore the dent. The skirt of the piston does go past there, but the rings don't come anywhere near it so a bit of roughness there isn't going to hurt anything.
If you're planning on unloading the truck, just drop in a used junkyard engine and be done with it. There's not a whole lot of a point to investing a ton of money into something you are only going to get trade-in value on.
Did you ever post the story that goes along with that bent rod, or is it just one of those things that happened?