I was told if I used 360 rods with my 390 crank it will cause vibration?? I was
told this by a racing machine shop guru. Personally, I don't see how this could
be because the rods look identical except for a little longer. I put the calipers on them and compared them they spec. out the same except for the length.
Any experience???

Here's what is going on. 1976 F100 truck 360 to 390 conversion. Using 1964 390 crank.

I ordered truck pistons for my 390 crank and rods and when I put a piston and rod in to check piston to deck height, I measured about .130 in the hole.
My 360 measured .120 in the hole and it didn't have a lick of compression.
So, I took a 360 rod with the 390 truck piston with 1.650 height. Put this
combo in and measured .075-.080 in the hole. Personally, this looks great
for my use. I would like to use this combo. But I am concerned about the
vibration issue. Any problems with vibration???? Is what he is saying so?

Thanks for all help, Duane.

 

As much as I hate to say this, the machine shop guy is wrong... All 8 cylinders being changed in the same way will not change engine balance.

My brother has this exact combo in his 1976 Highboy. When he rebuilt his 360 he put a 390 Crank in it. He used a cast 390 "Flat Top" (they just have the eye brows for valve clearance) pistons. It runs great, and like you noted does have a little higher compression. He did not have the engine balanced and the engine does NOT vibrate any more/less then my 390 or the original 360 that was in the truck. Another benifit to the longer rod is a longer dwell time at TDC, which makes more power... Of course the 360 rod is not much longer then the 390, therefore the increase in power is also pretty minimal.

Hope it helps, God Bless...

David

 

Yes, it will change the balance of the engine. When you balance an engine you make all the parts weigh the same as the other like parts. Then you weigh a complete piston assembly and come up with a bob weight. The bob weight it then attached to the crank and it is balanced from there. The 360 rods are lighter than the 390 rods by a good bit.

I know it doesn't seem like it makes sense but after I watched a guy go through the process I understood.

You can easily use the 360 rods in a 390 though. It can be easily balanced to work great.

 

I can't imagine doing a rebuild without balancing the assembly, which assures there is no vibration, regardless of the components. There is no vibration issue with 360 rods.

-Scouder

 

You should use the 390 crank and rods and order 360 pistons.....1.776 Cr height. Mic's to .016" deck.

 

EgoMan is right. I've built (4) 390s this way and they were screamers. I guess at my elevation though, it's less likely to detonate. I took one of them down to Louisiana for 18 months (Fort Polk) and man it ran.....after retarding the timing A LOT!!! Just BALANCE the rotating assembly when done, NO MATTER WHAT you've done to the engine. Just boring and installing oversize pistions throws your dynamic balance out the window.

By the way, I like the 390 rods better.....not that I've had durability issues with the 352/360 rods, but I like BEEF!!!


Cody

 

I have decided to just go with the 390 crank and rods and early model 390 pistons
1.759 height and dished out a little. This is the same pistons that they put in the
old thunderbirds, etc... If I figured the chart right it should give me about 9.5:1.
Which will be fine. This motor is going in an old 4x4 farm truck. It will never see
anything over 4500 rpm. Do you still think I need to balance the rotating assembly?

 

Id say closer to 9.0:1 cr not like it matters but its close enough and they should be fine.

 

If he uses 360 pistons with the 390 crank and rods, won't he hit the throws on the crank with the full skirt 360 pistons...??? 390 pistons have a slipper skirt design to clear the crank throws. I know a guy here that years back said that they used 360 pistons in a 390 and when they pulled the pan off, there were a bunch of piston skirt chunks in the bottom of the pan.

 

no, after market pistons for a 390 come in both sizes, its a common size now the (1.776"). So you can order them in the 390 style with the 360 comp height.

 

Probably 99% of 360 pistons used were the slipperier skirt variety. VERY few of 'em used the full skirt. Those were more for 361 FT engines. It's not like it'd be hard to tell something was wrong when putting it together either.

 

yes yo should balance the assembly it doesnt cost much and will help prolong the life of your engine, even if it is just an old farm truck they take some of the most abuse out of any truck. so anything to help it last is a good idea.

 

Balancing is a very good thing. Around here $150 will get you a premo balance job. include everything from pulley to pressure plate, rings and pin keepers. The only time I've seen negative results from balancing an engine was when a friend borrowed another friends mid 70's chev 454 powered tow truck. The engine was on it's last leg and went south while borrowing it. Being an upstanding guy he paid to have the engine rebuilt and had it balaced in the process. The owner of the truck was delighted by the new power. What we didn't know was when he had to tow a car on the side of the freeway he would pull off ahead of the car and put the truck in reverse, floorboarding it to speed up the time it took to get in front of the car. The first time he did this he made it a hundred feet or so before the engine started cutting out and clunked to a hault. We figure it was spinning 8K plus with the performance cam and valve springs that were added before the rods let go. From what I gather, carmakers used to use imbalance as a natural rev limiter before computer controls and such.

 

I've done two motors (390 and a Triumph TR7) without balancing them, and regretted it - the balance was so bad in the TR7 (not mine) as to vibrate the rear-view mirror so you couldn't see behind you - at almost all RPM's. The 390 was just as bad. Only thing changed was pistons being oversized .020 or .030"

From then on, I had the chance to do both types of motors over again (for myself, this time) and they are smooth as glass. All that vibration is not good for the mains especially, but also adds a lot of stress to everything else in the motor. Not to mention the power-loss from the crank trying to move the motor around in ways it shouldn't

Get it balanced, no matter what.

 

I agree, you should always have an engine balanced for longetivity and smoothness.