I have a good but worn out 390 motor, can anyone tell me what the maximum safe over bore is for this block. It is a 73 motor and will be used as a daily driver and off road 4x4 weekender.Thanks in advance .

"bowties in the rearview mirror"

 

the normal bore is 4.050 I think you can go to 4.130 IF you have the block `sonic checked` to see if it has the thickness in the casting.

.30 over is quite normal Ide stick to that


__JOHN__Õ¿ö
72 F-250
ICQ#6030753
http://clubs.excite.com/72FORD-F-250

 

Nod:
The max I would ever go is .040" over, but the other poster is correct. Stay with a max .030" and everything should be fine. This works out to 396 c.u. Personally, if it has to go more than the .030", I would get myself a 360 block and use that. If you can't find a 360, have your engine sleeved.
Also, since you are doing a total rebuild, make sure to drill the block (see the old Peterson Publishing book "Building your Big Block Ford), really check the valve guides, and get the replacement (427) oil filter adapter from your friendly Ford Dealer. They still have the performance windage tray too. That unit is good for 10-15 hp.
Good luck with your truck.
Skip4274
4everFord

 

maximum bore in most FE's is about 4.185,if you are going to run water in the block for cooling. I don't haveany idea where this 'thinwall' idea came from.This means 060 over 428 size is okay. on any 352 or 390 I would be comfortable going to 428 size without checking anything. Having spent some time working in an engine shop, I've seen 390's bored out to 4.25 for sleeve installation and there is no water showing. As best as I can tell, most FE's are cast at 4.36, the 427's are cast at 4.44. If we take my max bore and subtract it from the OD of the cylinder you will have just over .080 walls, thin but liveable.If you can find an FT block they are a little beefier. You will need a bushing from ford to use the FE distributer and oil pump shaft in the FT block. My060 over 390 is doing very nicely. Keep in mind there are some crazy fellows out there with drag race 390's that fill the waterjacket with hardblock and run 4.25 bore! Have fun.

 

well I find it hard to believe that you'd just bore a 390 to 428 and not care you probably read the article in the magazines a while ago on the homemade 428 out of a 390, they went through 6 390 blocks before they found one that had thick enough cylinder walls to support a 428 bore and even at the two of the cylinder bores had to be shifted slightly to keep the cylinder walls evenly thick. And there are a LOT of reputable FE sources that also say not to bore a 427/428 more than .030 or so over

 

Being a fan of these big old dinosaur engines for years, I have tried to find out just how thick are the cylinders, noone in my area has the ability to do sonic checks.I've done my best to guess at it. Here goes-knowing thet the bores are all 4.63 apart, I tried to measure the space between the cylinders,looking in thru the casting plug holes.All of the 390's&352's I checked had 0.27 or less between the bores. My 427 has about .150. If we subtract .27 from 4.63 it would seem that the O.D.of thebore is about 4.36.This would mean that you could go to 428 std bore and still have cylinder walls .115 thick-4.36-4.13 bore=.230 /2=.115 walls. Even 428 .060 over would leave cylinder walls.085 thick, but I wouldn't go any bigger. When installing a sleeve for crack repair on a 390, I took it to 4.25 and there was no water jacket showing. I realize this is only guesswork but unless some retired ford engineer can tell us how thick were they cast,what can we do? Have fun, and if you toast a block, at least they are still cheap in the boneyards. I have heard stories of thinwall FE blocks, but thought that only meant the 427. DinosaurFan

 

 

Thanks for all the information everyone, I'm going for a .040 bore and see what happens.

"bowties in the rearview mirror"

 

My machine shop said I could have bored mine .060" over... and I was going to, but they could only find .030 over pistons for it so that's what I did.



Garrett
[link:www.1966ford.com|1966ford.com]

 

if your machine shop can only find .030 over pistons for a 390 I'd start looking for another shop!!

 

Personally, I wouldn't bore a block out any more than what would be necessary to provide the new rings and pistons a smooth surface to run on. This way, there's still some room for another overbore when it's time for another rebuild. And the only way to properly figure out the max overbore for any block is to have it sonic checked. This invloves measuring the thickness of the cylinder walls at several key locations. Boring cylinders costs what, eleven bucks a hole?
I'd be awfully dissapointed to get seven bored and break into the water jacket on the eighth and have to still pay the shop $88 buck for a junk block-especially when I could have had it sonic checked for $25 bucks.
Jason Kendrick
1970 F100 Custom 390/C6
1978 F150 Ranger Lariat Supercab 460/C6