SC rods in a 4.2

[Roadhawg]

Will 3.8 SC rods work with a 4.2L crank and 4.2L pistons in a 3.8 SC block?

[matthewneuharth]

no.

first the rod is Way too short to use even if you had pistons made for that size. you would have to cut the crank or the pistons. or both.

you have to remember a Piston is Made to work with a certian size rod.

[Rookie]

Are the rods too short? Or would they be too long? I would think that because of the shorter stroke, the rods (in order for the pistons to come up to the proper height for compression) would need to be a bit longer.

Unless of course the pin location in the piston is lower for the 4.2.

Regardless, the rods wont work, although I would think that the 4.2 pistons with the 4.2 rods would fit on a 3.8 crank - it would be really low compression though... lol...

Eh - you're just better off using the right parts with the right parts.... obviously... hahahhaha...

[Roadhawg]

I guess I did not realize they changed rod length on the 3.8 in 1996.

Bore is 3.81"
Stroke is 3.39"
Rod is 5.9135"
Deck height is 9.232"
Compression distance is (top of piston to pin) 1.602"

In '96 Ford made the rods longer
'96 up rods are 6.091"
'96 compression distance is 1.450"

The 4.2 has the same bore but a 3.74" stroke
it uses the same rod at 6.091"

[Roadhawg]

What about this idea I found on another site.

As a theretical example if you used a 4.2 crank with a early 3.8 rod and a late 3.8 piston that assembly would rise 0.0015" above stock deck which is very safe.


Hmmm ....... but maybe the compression ratio would be far to high for a boosted engine

[Rookie]

That might be a bit high on the compression. :D Ok if you're going to run race gas, but for anything streetable on a regular basis that might not be the best...

[FR350]

If you had a custom set of pistons made you could use them but the pistons would be heavy and you would have to clearance the counterweights, I think is would be good for a pump gas n/a engine with a little compression but I would stay with a 6.125" or longer rod.

[Roadhawg]

I am going with a set of 6.125 Scat Rods, Ross Forged Pistons and a set of Total seal rings.

[mikael]

What about this idea I found on another site.

As a theretical example if you used a 4.2 crank with a early 3.8 rod and a late 3.8 piston that assembly would rise 0.0015" above stock deck which is very safe.


Hmmm ....... but maybe the compression ratio would be far to high for a boosted engine

It would actually be .0025" above deck, but the wrist pins are different diameter between early 3.8 rods and late 3.8 pistons.

And yes it can work using the SC rods, I know of one person who did it. Yes he had to clearance the counterweights a lot and get custom pistons. It worked out in the end and he makes good power. This was back in the beginning days of modding 4.2s for mustangs. Now there are a lot better options available.

[Crazy98]

I would go with the longest rod you can. The longer the rod the less force on the outside part of the cylinder. In the bikes when we stroke them we also go with what we call a long rod. If you think about it the longer rod will be in a more upright posotion when the crank throw is in the 3 or 9 oclock position. Less friction. less ovaling of the cylinder.

[Roadhawg]

............ I went with 6.125 SCAT H beam rods

[CobraSnakebyte]

:rofl:

[old_coot]

Just for some info--I just built my engine for my Super Coupe using the 4.2 crank, with the Super Coupe 5.915 rods with ARP bolts and H820cp Mustang replacement pistons. I had to shave the crank counterweights .200 but it still balanced fine also we had to resize the wrist pin holes in the pistons since they used a .905 pin and the SC rods were .912. the compression ratio works out to be right at 9.4 but thats about ideal for a roots type blower..I went this route since this is my daily driver and occasionally sees the dragstrip. I did not want to build a forged piston engine for daily use and put up with the rattling and oil consumption they entail. I still have the original 89 blower on it in a completely stock configuration--no CAI or larger TB or overdrive pulleys so it only makes 7lbs of boost but the dyno recorded 235HP and 320TQ at the wheels from 2000 to 4800 where we quit the run....I now have about 5000 miles on it and am getting ready to install the larger blower and overdrive it. I have been getting over 21 MPG in combined driving...the only mostly highway trip so far got a shade past 24 MPG....I'm loving it so far........Dan