I am fixin to buy some pistons for my 6.9. I found some that are .040 over, Mahle hypereurectic. the ones in the engine now are marked .040. I'm sure yall have seen the old pistons in another thread, they are BAD! My question is, what is the max limit for my cylinders? Are .040 pistons 4.040"? Or is the cylinder itself supposed to be 4.040"? I guess what i am getting at is, what should my cyliders mike if they are still in range for .040 pistons? I just want to make sure my block is still in range for .040 pistons before i buy them. Thanks.

 

Don't know the answer to your question, my shop did the checking for me. But are you getting your pistons of E-bay? They had some Mahle .040 for about $250. The had a .010 reduced compresion height which would work good with a turbo. I was going to order them but found a set with stock height for $130, couldn't pass up that deal.

 

I'll try to help and/or explain....
Your cylinders started out at 4.00 inches. If they were bored .040 over, your cylinders are now 4.040 inch.
Now, each style piston has different clearances due to their metals. Aluminum expands faster, so, I would guess a piston designed for a 4.040 bore cylinder would be around 4.035 COLD. Each manufacturer is gonna be different, but, you just have to trust their engineers. If you specify a piston .040 over, they already have done the research and engineering to know what to provide.
I have never dealt with your brand, but, if they are for a .040 bore, you should be good as far as fit.
I'm just not comfortable with boring a block that is thin .040 over without it being sleeved.
The 6.9 started out with 4.000 bores. Yours is now 4.040 bores. The 7.3, which is a bored out 6.9, has a bore of 4.110
If the SCA's weren't checked in your 6.9, it is possible that you have weak cylinders like a 7.3 has.

Simple math: 4.110 (7.3 bore) minus 4.040 (your bore) is .070 inch, meaning your cylinders are now .070 from being like the weaker 7.3 engine.
In another way to look at that is that the walls of a 4.040 bored cylinder are only about 9/128th of an inch thicker than a 7.3, or a little more than 1/16th of an inch thicker (8/128th is equal to 1/16th, so 9/128th is just a smidge bigger)

Your cylinders should mike out at 4.040 or slightly more. Your new .040 pistons would mike out in the vicinity of 4.025 to 4.035 to allow them to expand when hot to pretty well fill the 4.040 bore, minus the needed clearance.

 

I found a set on ebay for 150.00 @.040, just want to make sure they will work before i buy. I also forgot i have a ford factory 86 engine manual. It has all the specs in it. never really looked through it before now, it is really comprehensive. Tells you a lot more than my haynes book.

 

Thanks for the info. The block i have is already bored .040, so i'm stuck with it. unless i go huntin for another. I'm not sure about if it is sleaved or not. The engine is a reman from motorcraft. It all looks new inside, except for the beat up pistons. No sludge or buildup of any kind any where. I am running a turbo, so is it foolish to use this block. i'm not putting it on the dragstrip, but want it to be strong and dependable. So if i need another block, now is the time. Just would rather avoid the extra machine work and time and money. They are tight these days.

 

From my understanding, The late 6.9 and 7.3 non turbo were the same block. Only differance was they bored the block .110 over for the 7.3 and tapped it for larger head bolts. This being the case a 6.9 block bored .040 over should still be way stonger than a 7.3 standard bore. However the smaller head bolts make blown head gasket more likely, especially if turboed. I am putting ARP head studs in mine for that reason.

 

I already got a set of arp head studs, gonna be installed to be on the safe side. I think .o40 will be ok, as long as i don't have to go bigger. I ran that turbo on 7.3 for years with no problem. Till the dropped valve. I'm not planning on running the crap out of it, just having power when i need it, and lasting a long time.