FT "cobble"

[Shauncb]

Hello everyone,

I'm in the process of putting back together my 68 n700, the truck came with a 330 which is a good runner but needed manifold work so the engine came out. While the engine was out I picked up a 391 with a Clark 5 speed, the 391 smoked out of the valve covers like it had blow by but had no "pressure" behind it.
Anyways the 330 runs good and the 391 probably needs to be freshened up with I was to use it.

My question is... Can I swap the 391 crank into the 330 engine? According to my math that would make it into a 357 and raise compression some. I did the work/math and everything checks out fine. let me break it down.

the 330 has a stroke of 3.5 inches, the 391 has a stroke of 3.79 inches and that equates to 0.145 inches difference at TDC and BDC, I checked the 330 and The factory piston is down in the hole 0.165 inches so putting the 391 crank into the 330 would leave me .020 down in the hole which in theory would work. My next question is, does anyone know what compression would be as the 330 was originally 7.4:1? I also need to check on rod clearance with the added stroke.

I'm not building a race engine as it's still going to have FT heads and manifolds just looking for a little more power, it's not going to pull super heavy either maybe 20,000 lbs worth of cars at a time.
I'm also trying to do this on the cheap, the 330 was rebuilt at one time and ran good just had typical exhaust manifold issues.

Anyone done this before? Please don't respond with I'm wasting my time or tell me it won't work without any supporting evidence. I know this isn't the norm for this engine but what fun is being normal and following the band wagon??


Thanks for all the help and replies, I will be needing help with bearings and such as I'm not sure if the ft uses the same rod and main bearings as the fe.

[FrozenMerc]

I replaced a toasted 330 in a mid '60s F600 with a 390 out of a mid '70s F350 once, but I swapped the entire engine.  With hindsight, it may have been easier to go your route, then it was to sort out all the differences in manifolds, accessories, and mounts between the medium duty 330 and the light duty 390.  However, in the end the 390 worked out very well.

Your math seems sound, but I questioned the pistons being that far down in the hole to start, so I did some math too, and assuming a 72 cc head chamber volume, 0.040" head gasket thickness, and a flat top piston 0.165" down in the bore, you come out with a 7.06:1 compression ratio on the old 330.  Assuming there is a slight dome on the piston (say about 6 cc), or a slightly smaller head chamber (66 cc), you can get to the 7.4:1 quite easily, so I now trust your description. 

All that said, staying with the same geometry and dropping in the 390 crank should get you to a 0.020" piston recession (assuming no dome), but that comes with a healthy increase to an (approx) 10.42:1 compression ratio (assuming 7.4:1 starting point).  That might be a bit high, especially in a big truck application that sees lots of high load, middle to low RPM running, where detonation is most prevalent.

[Shauncb]

You answered questions I was hoping for, as far as compression ratios and such. These have a big dish in the piston. How big of a dish in the piston and what cc the combustion chamber is I can't seem to find. What about a thicker head gasket to take away some compression?

I appreciate all the info in this one post. 

[FrozenMerc]

One suggestion would be to use the shorter 391 rods as well, that will drop the piston another 0.052" down the bore and bring your compression down to 9.34:1.  This only works if the 330 uses the same 6.54" rod as other 3.5" stroke FE's, and the 391 uses the 6.488" rod.  If the 330 uses the shorter rod, never mind.  Then you will either have to find a 0.090" thick head gasket, or start milling a deeper dish into the pistons.

[frnkeore]

First, is yours a HD/XD 330 or a MD 330. The HD's have the 6.488 length rods.

Second, if you put the 391 crank in, I think you'll also need the 391 flywheel, for it's external weight. The HD 330, also has a forge crank and external weighted FW but, I don't know is the offset weight is the same. BTW, I have a HD 330, that's .060 over. It's not running, I bought it for the crank and to check out the block and cyl wall thickness. It's a later block, '73 date with outside ribs.

[Shauncb]

The 330 I have is an HD, I have the 391 Complete, swapping rods would'nt be a problem either, I just don't know if they are pressed rods or floating.

9.4 would be a pretty decent compression ratio and possibly a .050 head gasket would help also.

What is the proper way to measure a rod to verify the length?

[frnkeore]

With a std size rod bearing in it, add 2.439 + .975 together, devide by 2 = 1.707.

Measure between the 2 bearing holes, with calipers and add the 1.707 and that will give you the approx length. The dist should be ~4.781 for a 6.488 rod.