How much overbore is too much on a 4.1?

[Steve]

I bought a 4.1 off Craigslist for $50 the other day and I got around to tearing it down today. The motor does have a rolled fillet crank so that was good news but the less than good news is that it is already .030 over. The motor was running when removed and it has no major scratches in the cylinders so it wont take a lot to clean up but how much overbore is too much? I looked and Sealed Power has turbo pistons in .040 which would not scare me but they also have .060 which does concern me some. Anyone have any experience with a 4.1 with .040 over or greater bore? Thanks

Steve

 

[Duke]

get it sonic checked, dont guess

 

[charlief1]

I wouldn't go over a 4" bore whether you have it sonic checked or not.

 

[LOT]

Just wondering

I wouldn't go over a 4" bore whether you have it sonic checked or not.

What is your logic on this?

 

[hybrid71]

I've run one at .040 over (4.005 finished bore size) for over 10-years now with no cylinder problems. The rest of the block (cam bores, deck and main webs) had issues but the cylinders held up fine.

 

[charlief1]

What is your logic on this?

A 4" bore is a .035 over bore. The larger you make the bores of the cylinders the less stable the block can get. This doesn't mean it's unstable but I'd rather not take the chance myself.:biggrin:

 

[LOT]

answer

get it sonic checked, dont guess

There is your correct answer.

 

[Steve]

There is your correct answer.

That is likely the best answer but I don't want to spend more on the sonic check than I gave for the entire motor! I am going to drop it by a firend's machine shop and if it needs more than the .010 to clean up I am just going to chunk it. I bought a N/A 109 motor for $75 last night and its all standard so I can use the 4.1 crank in the 109 and be ready to go. Thanks for the replies.

 

[LOT]

wise move

That is likely the best answer but I don't want to spend more on the sonic check than I gave for the entire motor! I am going to drop it by a firend's machine shop and if it needs more than the .010 to clean up I am just going to chunk it. I bought a N/A 109 motor for $75 last night and its all standard so I can use the 4.1 crank in the 109 and be ready to go. Thanks for the replies.

I never did understand guys spending lots of $ on an old 4.1 (I know some people are having great luck--not me) with machine work. I know of no way to address the main weakness (thin deck) of the 4.1. Best of luck.

 

[hybrid71]

That is likely the best answer but I don't want to spend more on the sonic check than I gave for the entire motor! I am going to drop it by a firend's machine shop and if it needs more than the .010 to clean up I am just going to chunk it. I bought a N/A 109 motor for $75 last night and its all standard so I can use the 4.1 crank in the 109 and be ready to go. Thanks for the replies.

Before you chunk it, what 4.1 casting # is it?

 

[Steve]

Before you chunk it, what 4.1 casting # is it?

It is a 291.

 

[Ken Adams]

What are your plans for the 4.1? If you plan to sell, any idea what you will ask for it? I'm located in Central MO.

 

[Steve]

What are your plans for the 4.1? If you plan to sell, any idea what you will ask for it? I'm located in Central MO.

If I do sell it I will make you a great deal on it. I will sell it for whatever I am out on it at the machine shop which should be pretty little(likely just the hot tank fee and maybe an hour of labor). It is being hot tanked now and the machinist is going to check it out tomorrow so I should know the story then. Thanks

 

[jdpolzin]

I wouldnt worry about .030. My 4.1 is .040 and Hybrid71 ran a .040 making some BIG POWER!! TurboBob's 4.1 is .040 and he beats the snot out of his car. Like Mike said, he had more problems with the rest of the block.

Keep a good tune and it will last. Use a bad tune and even a Stage 2 will become shrapnol!

If you would feel more comfortable with a standard bore 109, go for it!

Are you planning to drag this car or is it a street bruiser?

 

[boostingbuick]

I have a 291 casting that I run with a 4.030 bore. Put down 550 to the tires. But I also had mine sonic checked before I went to that bore.

 

[Steve]

I wouldnt worry about .030. My 4.1 is .040 and Hybrid71 ran a .040 making some BIG POWER!! TurboBob's 4.1 is .040 and he beats the snot out of his car. Like Mike said, he had more problems with the rest of the block.

Keep a good tune and it will last. Use a bad tune and even a Stage 2 will become shrapnol!

If you would feel more comfortable with a standard bore 109, go for it!

Are you planning to drag this car or is it a street bruiser?

It will be a little of both. I was planning to build the motor like the motor that was in my WE4. My WE4 ran 10.70's on Nitto DR's with 60's only in the 1.6's. The motor is going in my 67 Firebird that is about 400 pounds lighter than my WE4 and should hook better(the Firebird has a better susp. set up). If the 4.1 will clean up at .040 over I will give it a try, but if it requires more than that I will likely go with the 109.

 

[jdpolzin]

I would have to agree with that!

 

[turbobob924]

$50 bucks!

Great deal. Get is tested before you do anything!

 

[dr_frankenstein]

i have one bored as far as .060, 291 casting but I did hard block the bottom inch of the jacket and had the block sonic checked first. thats the old engine in my truck..... has maybe 140K miles on it? still ticking.

A.j.

 

[hybrid71]

$50 bucks!

Great deal. Get is tested before you do anything!

Hey Bob,
What machine shop do you use that charges $50?
I just had a 291 checked at Opel (in Streamwood) and they charged $85.
I'm looking for a good shop in the area with fair prices.
Mike
Barrington