OK, so, here goes, and feel free to call me stupid, because I'm all about free speech, but at least tell me where I'm going wrong, but, I just recently picked up some ACL Race Series 3.9L Pistons, 6MKRY9390-0.50 for those who are interested, and I have a relatively unmolested EF block, will they fit and run?

I also have a 3.9 from an EB, so if they won't fit the 4.0 I'll just go with the 3.9, but it's more work for the 3.9 as it needs to be cleaned out as it's pretty well raped.

I'm after a weekend hobby, and an engine rebuild is just the ticket I feel. I've worked on engines before, having rebuilt 'yota engines for years (7A-GTE conversion, 3S-GTE, and the like), but the Ford engine is new to me. As far as I'm aware, and again I'm open to being corrected, the 3.9 block didn't have the same type of oil chambers to allow for oil to make it to the head at higher revs, something not corrected until the EF 4.0's, and the bore difference was 91.86 (3.9) vs 92.3 (4.0), but I read somewhere that the .50 ACL pistons would fit, or was that a crock?

you would need to measure your block and pistons and see how much room you have to move...
keep in mind you would need to at least hone the block which will take a thou or so off, providing the bores arent market. if you piston to bore clearance is suitable then you might be ok..

bottom line is doesnt matter what anyone says, unless you do the measurements you wont know, or if your unsure take the block and pistons to a machine shop and get them to measure for you.

ACL Race Pistons are measured in mm in oversize. Yours are stamped 0.5 oversize, therefore 0.5mm oversize or 20thou over size.

Bore on an EF 4.0 block is 92.26mm by memory and 3.9 is 91.86mm? correct me if I am wrong.

Therefore the EF 4.0 bore is 0.4mm bigger than the 3.9, so you could bore the 4.0 to suit.

Did you get the ring set with the Pistons?

Last edited by SWC on Fri Jun 12, 2015 9:17 pm, edited 1 time in total.

All info points to 92.3mm for the 4.0, and 91.86mm for the 3.9, so if what you're saying is right, then they should fit......... I'm guessing I'm just going to have a look I guess.