250 10:1 Pistons = AMC?

[powerband]

I'm investigating a Clifford Performance built Ford 250 six engine. Supposed to have Jahns 10:1 Pistons, Clifford rods, balanced bottom end, Fisher balancer, 1.88 int. valves, Ported and relieved, Offy 3X1, Re-curved distrib., the works...


I've been trying to find out what was done to accomplish the compression ratio. The pistons were not likely not Jahns sinc they have a D shaped sump in the head, I talked to Chris Neilson at VintageCams , who bought up most of the Jahns' s stock when they folded. I found casting numbers on the top of the pistons to be STD - 470NP. What I discovered is the pistons are likely to be TRW/SealedPower # 470NP's. The application is 79-89 AMC 4.2(258).

I've never heard of using the AMC's but the specs seem to bear out the idea:

Ford 250: bore = 3.68 / comp distance = 1.5 / dish volume = 13cc
AMC 258: bore = 3.75 / comp distance = 1.633 / D-Sump = .178(?)

This would put the bore at @ .070 over and raise the piston toward the deck .133 . The specs list the pin diameter of the ford at .9122 and the AMC's at .9310 which could easily be accomodated.

Powerband 8)

 

[MustangSix]

A .070" overbore is probably right at the limit without sonic testing, but is feasible. The CD would be about right for stock rods and the small end could be opened up.

The chambers would need some work to keep the CR within reason.

 

[powerband]

MustangSix

Rambler pistons? - That's a new one!...

That's the mystery : I could not figure how the CR was done. The AMC Pistons do not have a stated relief volume but the depression is much bigger than a 200/250 piston -I'd guesstimate 2 to 3 times the volume. Since the 200/250's range from 07 to 13 cc's - the CR must be lessened significantly and still maintain zero or negative deck height with the .133 additional compression distance.

The fun is that this engine has never been fired and I want to keep it intact as it was built by Jack Cliffords back when... - I hung a light bulb in the cylinder and read the piston #'s with a mirror.


Powerband

 

[CZLN6]

Howdy Powerback:

Wow!!! Thanks for sharing that discovery. I did a quick search on the Silvolite website and found a similiar piston. It is part #2227. They list the cc volume of the recess at 21ccs- which makes it ideal for this application. It lists the depth of the recess at .185".

With the .070" overbore the bore is 3.75", stroke of 3.91, a Felpro head gasket at .050", a zero deck height, chamber volume of 60ccs and a piston recess of 21 ccs, gives an ideal street compression ratio of 8.8:1!!!

And, the recess is not "D" shaped, but appears to more closely mirror the shape of the combustion chamber of a small ford six. The dish is more of a bathtub shape.

This piston has got to be more accessible to obtain than the 2.5 HSC rod route I've been pursuing.

Thanks for the tip.

Adios, David

 

[powerband]

David;
Thanks for adding some qualified credence to my determining for how the Performance 250 was built back in Jack Clifford's shop. I have been on this since I received it from the original owner who spec'd the build. I've been haunting my machinist and contacted the current Clifford organization as well as Chris Neilson and a few others with no results. The stamped STD I could see on top of the piston from the spark plug hole was fueling my curiosity.

And yes the relief is shaped exactly as you said rather than a D.

Finally I need to ask:

This piston has got to be more accessible to obtain than the 2.5 HSC rod route I've been pursuing.

I have been wondering since I first read the Six Cylinder handbook what the HSC means in reference to the 4cyl. often mentioned.

Thanks,
Powerband 8)

 

[CZLN6]

Howdy back PB"

"HSC" stands for "High Swirl Combustion". It was a PR promotion more then anything. The idea is that high swirl creates a more efficient combustion. It is what modern combustion chamber design is all about. It is what quench is about.

The AMC pistons you've got in your engine were used in an 232 ci and 252 ci AMC six cylinder engines from 1979 to 1990. We'll be searching for piston weights and pin types next. The recess is very close to what I envisioned CNC milling into the top of a flattopped 2.5 HSC piston. If the AMC pieces work, that will cut down on the cost considerably.

What are your plans for this gem? When will it run?

Adios, David