hey fellas,
I've began a build of a 68 mercury 390 to put in my 76 F350. Its a daily driver and i want great compression for best mileage and street power i do like to have more 'go fast' for this truck.

Right now the block is .030 over with what look like sealed power dished pistons if i remember right they are a 304np? seem like a std type piston and would yeild fairly low compression.

what would these pistons set me up with now as far as a compression ratio and should i go for some '360' pistons since they are of a bigger compression height and technically a flat-top? OR should i go as far as getting a dome topped piston to yeild more.

Im wanting at least a 9.5 to 1 or 10to1 compression unless ya'll have a better recommendation?!?

thanks everyone

 

For '68 you had the 2bbl "Regular Fuel" 390 with the slight dished piston and valve reliefs like you describe. Ford claimed 9.5 CR.

You also had 4bbl "Premium Fuel" 390s with flat top pistons with valve reliefs. Ford claimed 10.5 CR. There were also 2bbl "Premium Fuel" 390s, same piston.

The "360 piston" is either of the two, according to posts here. Folks have found both the flat top and slight dish. The shorter stroke of the 360 leaves the piston .100+ down the hole, despite the slightly longer rod. (compared to .010-.030 on the car 390s)

In the truck 390, the '66-'67 410 piston is used, and ends up similarly down the hole for low compression.

IMO the piston you have, or a "360 piston" with a slight dish is the way to go.

FE Series Engine Specification Chart

Note those '68 heads won't like unleaded unless hardened seats have been installed, but you knew that.

 

HAHA yes i do know about the valve seats, the hard way. The block crank and heads i took directly to a napa machine shop right after buying the engine(which was already disassembled...such a PITA) i had them install cam bearing which i supplied, vat the block and do freeze plugs IF they had any i wasnt worried about it, vat and polish crank, vat and whatever else to the heads.

ended up all the seats had to be replaced along with guides, which the machinist told me i was up to 500 in labor alone when he had one head complete and the crank was in vat as he was on the phone with me. now theyre telling me 685 for total bill im going to have to get a loan to pick this bstrd back up!

this engine did come with the 2bbl intake.

the thing is that the man i bought this from said it was installed in a 68 truck when he recieved it(he gave me the 4spd T18 along with it) but the valve covers are mercury and the hose setup on the intake looks like it had some crazy emissions like a car would versus a truck. Im ASSUMING its a mercury engine but one thing i noticed is that the mains have the same type webbing as my 76 f350's 360 engine(dtea?dt4~? block casting). i had the casting numbers handy but that napkin went POOF somewhere. every cast# ive ran into starts with C6-- so i know its 60's. also came with the fuel pump w/ filter in it and a dual vacuum points dizzy. I was cleaning parts today and i also have a one piece fuel pump eccentric that is really thick while my 76 360 has a two peice. IDK if these details scream anything to you? it sounds like you've got a lot of this down pretty well.

Another weird thing is that while cleaning the main caps the rear seals were still in that cap and looked like brand new with C6... FOMOCO numbers still on them and the little side seals had what look like nails or sharpened wire hangers in them? there was red RTV all over and clevite 77's 10 over so i know its been built before. Ive never seen this before but this is my first time THIS deep into an FE.

I didnt get a very good look at the block which had bearings still in it but the caps i have with bearing also and they've all bronzed and scored like i said were .10's but the mains have oil holes in them. Im not for sure if both bearing halves have oil holes otherwise whoever build this thing put them in backwards! the bearing halves i have look oil starved bad. I have the M57HV pump that was with it also.

WOW sorry about all that info hope i didnt go on too much!
thanks 85e150six4mtod!

 

I wouldn't read too much into what parts are there or not. Especially since it's been rebuilt.(and it's a 40+ year old engine) The Merc 390 used the same parts as the Ford 390. Only thing different was the high comp 2 bbl motor in the Mercs, and then that was just the use of the 2 bbl intake in place of a 4 bbl. As for the comp ratio, you can look up the piston part numbers and find that part out by doing the math with the comp height. The side seals on the rear main are supposed to have the pins.

 

I got the H395P and they were supposed to be 9.5 but if the chambers are on the large side it could be 9.3 IIRC.
puttster

 

Power gos with the higher compression as the motor plus the weight load you'll never be buying regular gas for that beast. 89,91+ octane gas and the price at the pump these day has stopped a lot of fun cruising times. As my last motor was a 67 390 car engine. Never got over 10 mpg at best Had to run the timing down to 0 TDC-2*ATDC
whit out pinging . Now sold the camper changed to a new motor with 8.7 ratio running regular cheap gas of 4.29 a gallon now so something to give some thought..
orich

 

You could have fixed the octane requirement problem by simply swapping the cam. I've built two 10 to 1 390's with mild cams that were fine running 87-89 octane gas. The first had a Cam Dynamics grind that was (been a long time, so these figures are not hard) in the 290-296* advertised range and a .516-.523 lift. The second was a stock pistoned 68 Merc 390 with a Crane 272* Energizer, it was fine with 89 octane and 12* initial timing (stock distributor and a Pertronix II)

 

As noted in my post, I had a over head camper on that was 1850 lbs+ gear400 lbs & truck itself 5,525=7775+ wife & me= 8095 not counting wt. of any gas all moving like a brick wall down the hwy 8-10 mpg @50-55 mph. But if just a never loaded cruiser or toy like most classic trucks are use for today.. Plus your getting better then 12 17 mpg and don't mind putting out for the higher octane gas when regular is at 450-5 bucks a gallon go for it and don't look back as you got power. It sure talks the fun out of driving these old classic tho! I felt the pinch for 30 yrs on having to buy the highest octane gas every yr on vacation. There's is no ez fix no going back once done unless new pistons..Just putin in my 2 cents to think of what your needs are and not what your wishes are..
orich