Strange Vibration at 1200 - 1400 RPM in V8
Strange Vibration at 1200 - 1400 RPM in V8
(OP)
I'd appreciate some engineering perspective on a problem my engine has had for a while (ever since I've owned it). I'm not looking for normal mechanic-type troubleshooting advice, people intimately familiar with these engines have weighed in and come up with nothing. It is very unusual.
My '52 Ford truck's flathead V8 has a vibration at the specific RPMs listed, and only there. The vibration seems like a couple fore-aft, rather than side-to-side, as it wags the alternator that way (which is mounted rather high in front and on a 3/8"-thick plate) visibly -- think "pendulum".
I've had the engine nearly completely apart; have not removed the crank. All bearing clearances are in spec. The flywheel has been balanced with the clutch mounted. Compression is uniform within 5 - 10 psi across all cylinders. All plugs are burning clean. Oil pressure is great, about 50 psi at any speed above 1,000 RPM, cold or hot. I have taken off all drive belts and it has no impact on the vibration, so it is clearly something inside the block.
The only things I've found are:
1. one of the connecting rods is not original. There are two of them stamped as #2, with the odd one at the #3 position. All are the correct piece for the engine. Since the engine was rebuilt in the '70's, I'd suppose the #3 was replaced for some reason with another stock piece. No indications on the bearings of a twisted or bent rod, all pop up to the same deck height.
2. cylinder wear is at the limits, tapered from .005 - .009". These use 4-ring, long-skirt pistons with fairly high tension by modern standards.
These engines are a 3-main-bearing design, 6.8:1 compression ratio, side-valve arrangement. I did not know this problem existed when I had the engine apart, so I didn't weigh the piston assemblies, it was a clean-up operation.
My question really is, is vibration at this range indicative of a specific type of problem, e.g., a once per rev harmonic? A bent crank? Could it be pistons rattling in the bores? I'd love to know what to look for before resorting to a complete teardown and rebuild. These are not cheap to rebuild. Other than this vibration, the engine runs great.
Any ideas would be welcome!
My '52 Ford truck's flathead V8 has a vibration at the specific RPMs listed, and only there. The vibration seems like a couple fore-aft, rather than side-to-side, as it wags the alternator that way (which is mounted rather high in front and on a 3/8"-thick plate) visibly -- think "pendulum".
I've had the engine nearly completely apart; have not removed the crank. All bearing clearances are in spec. The flywheel has been balanced with the clutch mounted. Compression is uniform within 5 - 10 psi across all cylinders. All plugs are burning clean. Oil pressure is great, about 50 psi at any speed above 1,000 RPM, cold or hot. I have taken off all drive belts and it has no impact on the vibration, so it is clearly something inside the block.
The only things I've found are:
1. one of the connecting rods is not original. There are two of them stamped as #2, with the odd one at the #3 position. All are the correct piece for the engine. Since the engine was rebuilt in the '70's, I'd suppose the #3 was replaced for some reason with another stock piece. No indications on the bearings of a twisted or bent rod, all pop up to the same deck height.
2. cylinder wear is at the limits, tapered from .005 - .009". These use 4-ring, long-skirt pistons with fairly high tension by modern standards.
These engines are a 3-main-bearing design, 6.8:1 compression ratio, side-valve arrangement. I did not know this problem existed when I had the engine apart, so I didn't weigh the piston assemblies, it was a clean-up operation.
My question really is, is vibration at this range indicative of a specific type of problem, e.g., a once per rev harmonic? A bent crank? Could it be pistons rattling in the bores? I'd love to know what to look for before resorting to a complete teardown and rebuild. These are not cheap to rebuild. Other than this vibration, the engine runs great.
Any ideas would be welcome!





RE: Strange Vibration at 1200 - 1400 RPM in V8
Drive the bitch till she breaks.
Mike Halloran
Pembroke Pines, FL, USA
RE: Strange Vibration at 1200 - 1400 RPM in V8
RE: Strange Vibration at 1200 - 1400 RPM in V8
The stock rods are often swapped and most modern rebuilders use French rods or custom. My '53 59AB was finished Thursday the 20th. No vibrations on the test stand, but it will be fitted with a tq converter to C4, so no flywheel/clutch to bother with. Your correct about the cost, the flathead has become an "exotic"...$7909 with my stock crank and rods.
Before a complete teardown, maybe try another flywheel or ck runout on yours. You said it was balanced? Stock flywheels are a dime a dozen around here, no one uses stock any longer. Other than that, it sounds like it's time to take the crank out!!!
Common problems in a flathead were blown head gaskets...Worst case, a cylinder would hydraulic and bend a rod. However, you said all pistons came up equally. ??? Besides, I don't think that would cause the vibration you speak of.
I will pick up my engine at H&H next week, I'll ask Mike about your vibration. He's the expert these days.
When your tear it down, let us know what you find. Good luck.
Rod
RE: Strange Vibration at 1200 - 1400 RPM in V8
Joe Abbin of Roadrunner Engrg is here in town and I had him listen to it, he was puzzled, too. What puzzles me is the RPM it occurs at, it seems low for most kinds of balance problems, which I think would include a bent crank nor a bent crank flange. I'll check the runout, although it was surfaced correctly, referenced off the flange area. The balancer removed material 180 deg. from where the factory did... always seemed suspicious to me.
RE: Strange Vibration at 1200 - 1400 RPM in V8
I had it done to look like the 4th and 5th engine on the H&H site. All black. Offy heads with just the tops of the fins polished and chrome acorn nuts. All the bells and whistles on the inside, reground cam, adjustable lifters, Manley valves, etc. My core was a '53 out of a wagon that only had ~60k miles on it. Actually, I could have used it as is, but I wanted something a bit better. I sold my Metropolitan for $8k so, that was the budget.
Give Mike a call, he's a nice kid. He is really into flathead V8's. One on the bench w/supercharger and bling was $28 large!!!
Rod
RE: Strange Vibration at 1200 - 1400 RPM in V8
http://flatheadv8.org/phpBB/viewforum.php?f=6
RE: Strange Vibration at 1200 - 1400 RPM in V8
RE: Strange Vibration at 1200 - 1400 RPM in V8
Rod
RE: Strange Vibration at 1200 - 1400 RPM in V8
<<<
... and to me.
Mike Halloran
Pembroke Pines, FL, USA
RE: Strange Vibration at 1200 - 1400 RPM in V8
Rod
RE: Strange Vibration at 1200 - 1400 RPM in V8
Franz
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RE: Strange Vibration at 1200 - 1400 RPM in V8
Is this your setup?
https:/
I'd run it for a minute with the alternator belt removed, then with the alternator heavily braced, and then with the alternator removed.
Points triggered ignition? I'd watch the ignition timing and dwell to see if something goes unsteady in the 1200-1400 rpm range. Also manifold vacuum. I might cup my hand over the carb throat or pull the choke slightly when it goes unsteady to richen the mixture, just to see what happens.
My preference is to ID the frequency of the vibration.(s)
If mechanical unbalance is behind it, the vibration will be 1X. The opposite is not necessarily true.
A real vibration analyzer is best.
A reed tach like this might be enlightening.
http://ww
RE: Strange Vibration at 1200 - 1400 RPM in V8
No harmonic balancer, plain old cast iron sheave.
The clutch and flywheel were balanced individually, then together, with clear markings for reassembly. He added two pieces of 3/8"-dia. rod, a little over an inch long, to the clutch cover to get it balanced. I also just realized that this vibration occured with a completely different clutch assembly (running a diaphragm clutch now, previously had a Long-type stock clutch).
Yes, that slingshot is what mounts the alternator. I used a shorter belt in an attempt to get it lower (less pendulum), no difference. I have run it with no belts at all, no difference in RPM that it occurs at or in amplitude. I have wondered if the alternator is picking up a vibration and amplifying it? But this type of mount is very common and no one else is complaining about vibrations.
Tmoose, I am not clear on what the reed tach would show?
RE: Strange Vibration at 1200 - 1400 RPM in V8
If the big vibration is at crank rotating frequency, unbalance is a suspect. If the predominant vibration is at some other rpm/frequency, then the diagnosis changes
RE: Strange Vibration at 1200 - 1400 RPM in V8
RE: Strange Vibration at 1200 - 1400 RPM in V8
RE: Strange Vibration at 1200 - 1400 RPM in V8
Here's a spectrum analyzer, used to be state of the art for balancing Helicopter rotors & propellers. Now coming on the market fairly cheap, I paid $1000.00 for a full kit. Hook a couple of velometers up, and run the engine. Little plotter shows the peak IPS vibration and rpm where it's occurring.
http://www
no financial interest
RE: Strange Vibration at 1200 - 1400 RPM in V8
RE: Strange Vibration at 1200 - 1400 RPM in V8
RE: Strange Vibration at 1200 - 1400 RPM in V8
RE: Strange Vibration at 1200 - 1400 RPM in V8
I am anxious to completely remove the alternator and run it. The slingshot-type mount these use is 3/8"-thick plate. The stock generator mount is a casting that is deeply ribbed to stiffen it against exactly the type of wagging I'm seeing. I've previously only removed the belt off it. It is possible it is picking up a natural fore-aft force and resonating. If that is the case some stiffening ribs can be welded on.
RE: Strange Vibration at 1200 - 1400 RPM in V8
Even if the alternator (and its mount) is not the problem (and I suspect it is) - surely that mount is very poor engineering practice - I am surprised it works at all.
RE: Strange Vibration at 1200 - 1400 RPM in V8
I.e., the source is elsewhere.
Mike Halloran
Pembroke Pines, FL, USA
RE: Strange Vibration at 1200 - 1400 RPM in V8
RE: Strange Vibration at 1200 - 1400 RPM in V8
To make a long story a bit shorter... I found an old 50's/60's Kellison bodied dirt car from Oregon. The "real deal"... Too much to resist. First engine was a Esslinger Pro4 which I promptly broke. New engine will be my '53 flathead V8.
My son's an automotive artist and did the paint and body work. I did the grill from a raw sand casting. Frank's Hot Rod Upholstery of Temecula did the camel leather. H&H Flatheads did the new engine.
I should have it running again late summer.
http://www.youtube.com/watch?v=1uspjugZUKU
Rod
RE: Strange Vibration at 1200 - 1400 RPM in V8
As soon as I hefted the alternator out of the bracket I figured that was the case -- it isn't light (GM internally regulated 100-amp unit), at least 20 lbs. The bracket is simply not able to resist fore-aft vibration despite being 7/16"-thick. So I will weld a flat bar 1" deep along the edges of the slingshot to give it more stiffness fore-aft, and if necessary add a brace back to the intake manifold.
Thanks for all your help!
RE: Strange Vibration at 1200 - 1400 RPM in V8
Lots of folks have had several bouts of expensive work done on driveshafts AND wheels-n-tires trying to kill a frustrating vibration. But 10 minutes work with a vibration analyzer usually clearly picks up wheel speed ( around 800 rpm) or driveshaft speed (2000-3000 rpm) and pretty much eliminates one group from the lineup.
RE: Strange Vibration at 1200 - 1400 RPM in V8
RE: Strange Vibration at 1200 - 1400 RPM in V8
RE: Strange Vibration at 1200 - 1400 RPM in V8
Still, like others, I doubt that the alternator itself is the actual problem. But, you can stop it from amplifying the vibration which could make the vibration acceptable.
RE: Strange Vibration at 1200 - 1400 RPM in V8
Running without the belt pretty much eliminated all those items.