deck height

Edited 11-12-19 to delete my extra long comment..

The idea of using 4.0 rods won't do anything for the performance level of the engine -all the stock rods are made of the same nodular iron material -basically all the same strength.

For AM inline sixes, I'd say 'keep the stock factory balanced rotating assembly and stick with all the traditional bolt on hop up parts' -This will be a better investment of your hard earned money, you'll get a better running engine and more bang for the buck.

(sorry to put Farna into a discussion on balancing the engine) 


Edited by amcenthusiast - Nov/12/2019 at 12:51am

The only reason to use other rods would be if you wanted a different length rod to run a different piston from stock. 
Best to concentrate on a good valve job, porting, a good choice of cam, etc.

The 4.0 rods have the exact same bottom end as the others - I know - I've got early AMC rod bearings in my 4.0. (ok, so there's one difference - the locating tab was moved from one side to the other with the 4.0 so I filed new notches in the rod caps, etc. otherwise they are identical)

There's no reason to NOT use them - that part I don't get as to the negative fuss about "no, don't do it". Uh, why not, obviously depending on the PISTON choice, etc.
If they are the same length center to center, the same pin diameter (I've not checked that), then why not? They are sure easier to find if a person needs rods. 
But there's no benefit other than availability.
If a person uses different pistons - different pin height, there's all sorts of things a person can do. 
Rod length is something best left to professionals to deal with, though - not taken lightly. 
Stick with stock rods unless you plan on any other tweaks - pistons, funky crankshaft combo, etc.

Farna is talking of the way the crankshaft is designed and the firing order was chosen to prevent torsional twisting and harmonics. As a cylinder fires and the piston pushes the rod which pushes the crankpin, there tends to be a bit of a flex or twist in the crankshaft - a "wind-up" and harmonics, a kind of jump rope out of control situation where the crankshaft is winding up, unwinding, etc. and anyway, it gets complicated. Don't mess with the design other than choosing the right parts for the goal in mind. Set a goal - how will you drive it, what do you expect out of it, how will it be used. Buy all parts with that goal in mind and keep in mind it's a SYSTEM, not a collection of random individual parts. If you decide to go different exhaust - perhaps headers, then you have to consider what's going in and how it's getting there or the headers will be a bolt-on joke. Want more carburetor? Unless you change cam and exhaust it may not matter (depending on choice)
Want some fun - go the Holley EFI kit - no messing with a carburetor, no choke problems. Carburetors are fine - not a thing wrong with 'em but that kit would make sure the engine was getting peak air/fuel ratio regardless of what else you do.