Slant 6 balance

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Moparbrad

Moparbrad

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Hello! So I’ve been working on my recent acquisition, a 1968 Valiant. It originally came with a 170 slant six engine and a three speed manual column shifted transmission. At some point, somebody put a 225 engine in the car. It has a shake at idle and a harmonic vibration at cruise speed. Looks like they reused the 170 valve cover, intake and exhaust assembly, oil pan and what looks to be the harmonic balancer. I only say that because all of the 170 parts are red and the block and head are black. I don’t know what year the 225 is, it has no identification stampings on it anywhere. It just has a casting number on the drivers side of the block that says 030–11. Question is, does the 170 and 225 use a different harmonic balancer??? Also, could the flywheel be incorrect as well? I know the 170 and 225 have a different register hole on the flywheel, but is the balance different? Any help is appreciated!
 
They are all internally balanced to the best of my knowledge. It's possible it has a clutch or pressure plate problem. Have you checked to VERIFY that it's firing on all cylinders?
 
Yeah, it’s firing on all 6. I just tuned it up and double checked everything.
I read there is a cast crank slant and a forged crank. Could that be an issue?
 
Moparbrad said:
Yeah, it’s firing on all 6. I just tuned it up and double checked everything.
I read there is a cast crank slant and a forged crank. Could that be an issue?
Click to expand...

The cranks will not interchange with their respective blocks. But as far as I know all the balancers, flywheels and flex plates are interchangeable. Maybe @slantsixdan will come in and verify this. I am almost 100% certain this is correct. I tagged him, so give him a little bit. He'll chime in.

I left one thing out. The flex plates and flywheels and converters are not interchangeable between early and later models. I forget where the split is. It has to do with the size of the crank register. I am sure Dan will enlighten us about that as well.
 
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I'm currently removing the slant six from my car, I have a used clutch as well as harmonic balancer (and a bunch of other parts) you're welcome to if you need them. PM me if you want to talk about it more, I'm in vegas so only a few hours from you. Mine is a 225 as far as I know...
 
If it follows LA crank register size change, its 68 and up with the 1/4 larger bore up to 68 was smaller bore. As for balance of a 120 degree 6, there is none externally as i remember. The design of a 120 degree straight 6 is 'natural' meaning of the crank is undisturbed and the piston and rod packs are all neutral balanced, there is no external balance needed. Like a 4 banger as long as the pistons and rods match, its self balancing. Most slant 6 dampers have a straight line cut out the back side. Not sure what this is for but they all seem to have it. V8 is a lot harder to balance as the re are other forces at play being a 90 or 60 degree offset. 5 cylinder inlines are just weird. I think they are flat plane 180 cranks but the turbos dominated the rally world in the Audi Quattros I think the were classed out.
 
Are you sure you got all the valves set correctly? One cylinder a little tighter or looser than the others will cause a shake.
 
Unless the engine is a "Frankenslant" that has been assembled with mismatched parts, I would suspect the clutch. If you can swap in another runner, that would be an easy way to troubleshoot the issue (I know an engine swap sounds drastic). To have your current engine balanced, you would need to pull it anyhow, completely disassemble and take the reciprocating assembly to a machine shop. The balance job will be $200 or so, plus a complete gasket set, rings, bearing inserts and timing chain since you're in there anyhow. This could eat up $500 really quick with you doing all the labor.

Later model slants don't have spark plug tubes. Not sure what the change year was.

If you're driving along at the speed that the vibration happens, and you pop it into neutral and coast letting the engine drop to idle, does the vibration go away?
 
Just to verify the valves and ignition for each cylinder is ok, I would start it up and pull off the plug wires 1 by 1 and see if any dont change the roughness more. If so, that cylinder isn’t firing. Hopefully the cause is nothing real bad like a broken rings, piston, bent valve etc. As far as the possibility of being out of balance from the wrong parts is very unlikely. Pishta brings up an interesting possibility that the flywheel center hole may be the large one on a small hole style crank that happened during the engine swap.
If the plug wire thing doesn’t show any evidence, I think your time and money would best be spent pulling the bell housing back and look for anything obviously wrong, loose or damaged or off-center pressure plate, disc damaged, bad throw out bearing, wrong pilot bearing and including that flywheel crank interface mentioned above.

please keep us posted on your progress.
 
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