I remember the first time that I came across a 391FT engine. There was two trucks (1975 & 1976), abandoned up in a field in back of this man's house.FE series parts I could get at the time.

When I first saw these engines I was confused! The engines appeared somewhat like the FE series with the same valve covers and the simular intake/head matching design as the FE series but other things appeared different (timing chain cover, water pump and etc). I later learned that the two engines belong to the FT Series engine family.

About the only things that are directly interchangeable with the FE engine family are the rods, lifters, rocker arm assemblies, push rods, and valve covers.

The FT engine stands for “Ford Truck”. They were a heavy*duty engine option on some F250/F350 and larger trucks. The stock cylinder heads featured 1.75” intake valves and 1.50” sodium filled exhaust valves with positive valve rotators. Combustion*chamber volume was 78.5*81.5cc. The 361/391FT was designed for low*end torque and low rpm operation, ideal for their application.

Some Facts About The 361/391FT Engines

The 361cid has a 4.05" bore and 3.50" stroke.
The 391cid has a 4.05"bore and 3.79" stroke.

The 361/391FT blocks are heavy-*duty units and are virtually identical to the
FE except that the distributor pilot hole is larger to allow for the 5/16" hex
oil pump driveshaft, used by the heavy*-duty, high capacity oil pump and the
casting features 4*bolt engine mount attachment pattern.

There is a very good chance the 361/391FT block has the heavy*duty
reinforced casting. These have extra webbing and heavy cylinder jackets
which allow eventual boring to 428 numbers (including 428 + 0.060
overbores. The "105" blocks were cast at Ford's Michigan Casting Center.
Most "105" blocks have the heavier cylinder jackets and have reinforced
main bulkheads commonly found with 361/391FT and 406/428FE engines.

The FE series blocks were near end of production by 1972, so in 1973, Ford often substituted these blocks in place of the standard FE blocks until the end of 1976 in pickup trucks. A bushing was installed in the block' distributor pilot hole so that the standard FE series distributor would work. So, yes, the FT block will work as a substitution for an FE casting.

The rods, though they may have the truck number forged into it, are the same as 390/410/428 car rods, and are thus very desirable.

FT engines featured a dual roller timing chain and sprockets. They
also had a single front mount that attaches to the steel timing cover.

The 361FT came with a 2V Holley, and the 391FT came either with
the same 2V Holley or with a 4V Holley. Both were governor
carburettors, had a tall tach*drive governor distributor, truck* only steel
front timing cover and dual thermostat housing. The 361/391FT
featured a full* sump oil pan (8qts) and special pickup.

The 361/391FT series featured 4*ring dished cast aluminium pistons (7.4:1CR) and use short connecting rods. The 361 engine had full skirt pistons while 391 engines featutred slipper skirt pistons. However, I have seen slipper skirt pistons in some 361 engines.

The 361/391FT cylinder heads had the exhaust heat crossover setup completely different from FE units. (If you take the exhaust manifold off there will be 5 holes instead of 4.) The exhaust heat crossover on the FT cylinder heads went straight through the head from extra holes in the center of the exhaust manifold. The FT heads had 10 exhaust manifold bolts per head, whereas a car or light*duty
truck engine had 8 exhaust manifold bolts per head. The FT heads were small* valve low* compression 78.5*81.5cc) units that were unique to the FT series.

The FT intake manifold had a different exhaust heat crossover passage than the FE series as well and featured a dual thermostat setup.

The exhaust manifolds were a ram's* horn design that will physically
bolt up to an FE, but again the exhaust heat crossover port is
different than the FE series.

The 361/391FT featured a forged steel crank with a 1*3/4" crank snout, while the FE series had a 1*3/8" snout, and therefore used a different balancer and
pulley setup. The dampers of the 361/391FT were counterweighted. The FT series were externally balanced.

On a 391 crankshaft the first two counterweights (closest to the snout) will be in line with each other;they are offset from each other, then it is a 361. The 361 engine had a full* circle rear flange with no indexing notches or half moon cutouts.

If trying to identify a crankshaft that's already removed, set it on it's rear flange and look down at the #1& #2 journals. If it looks like 'Mickey Mouse ears' or a hat it's a 361. If the 2nd, 3rd, & 4th journal is rounded or smooth, it's a 391.

The 391 crankshaft had a 3.786" stroke +/* .004" tolerance. The rear of the FT crankshaft was different than FE series as well in that the pilot hole was larger, as is the area to support the flywheel.

361/391 Journal Diameters Main 2.7479” Rod 2.4381”
361/391FT piston to ring width #1 3/32” #2 5/64” #3 5/64” Oil 3/16”
361/391FT piston* to* bore clearance was 0.0025*0.0033
361/391FT wrist pin* to* rod bushing clearance was 0.00015*0.00045
361/391FT valve stem* to* side clearance was 0.0010*0.0027 for intake and 0.0020 0.0034 for exhaust
*361/391FT intake manifold port dimensions: 1.84” x 1.34”
*361/391FT cylinder head port dimensions: 1.69” x 1.16”
* measured at cylinder head to manifold mating surface

Hope this gives a better understanding of these engines.

 

I have a 1973 361. It does not have dual thermostats. in fact sourcing a thermostat has proved quite difficult as I believe NAPA is giving me the wrong ones. The T stat housing has a bypass leading to the waterpump. I know on the 6.5 diesels this requires a special thermostat that blocks flow to that bypass when hot. The tstats I am getting from NAPA look like regular tstats. I had one form a 3208 Cat which looks like the right one as it has the block off for the bypass but hesitant to use it until I can confirm. Sorry for different font. Can't seem to turn it off.