Strange Vibration at 1200 - 1400 RPM in V8

[RossABQ]

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!

 

[MikeHalloran]

I didn't notice that my LT1 had a vibration at 2000 rpm until my friend Ted pointed out that his did. It never got worse, or better. It may not be possible to counter _every_ imbalance that exists in an engine, especially with a three-bearing crank.

Drive the bitch till she breaks.



Mike Halloran
Pembroke Pines, FL, USA

 

[RossABQ]

Maybe I should clarify, this is bad enough to rattle the entire exhaust, all the connecting linkages, even the pedals. It hasn't gotten worse or better in 3 yrs, but is not typical of these engines. With their low compression, huge flywheels, they are usually silky smooth.

 

[evelrod]

Ross, what you describe is decidedly not common. My first engine was my '49 Merc. With the lack of skills as a kid of 18 it came out smooth as silk, even with all the mods I made. The only problem I had with vibration was a bent (?) 1st motion shaft (input shaft) in the trans. It was enough to shake the whole car but stopped when the clutch was depressed.

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

 

[RossABQ]

" Your correct about the cost, the flathead has become an "exotic"...$7909 with my stock crank and rods.." -- YIKES! Did you pop for a blower, or lots of eye candy?!?! No question H&H is top drawer.

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.

 

[evelrod]

No blower it would not fit the image I want in my 23. Exotic is right. I build modern (well, sorta modern) engines here at my shop, but it was actually cheaper for H&H to rebuild it than it would have cost me just for the parts...Never mind all the hassle of running all over chasing parts.

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

 

[RossABQ]

PS Rod -- join up, we'd love to have you!

http://flatheadv8.org/phpBB/viewforum.php?f=6

 

[RossABQ]

Very nice TrackT -- and is that a Dominator I see behind it??

 

[evelrod]

1948 Norton Inter/ES-2, Ex Bill Young, four time TT champ with the Norton Manx team. Hopefully I'll have it up on Ebay soon.

Rod

 

[MikeHalloran]

>>> The balancer removed material 180 deg. from where the factory did... always seemed suspicious to me.
<<<

... and to me.



Mike Halloran
Pembroke Pines, FL, USA

 

[evelrod]

Hmmmm. Yes, especially if the "clutch and flywheel were balanced together"...AND then the balance marks were not aligned.

Rod

 

[franzh]

I agree with the strange weight removal. Its been too many years to remember if the front pulley/balancer is rubber sleeved, and thats where I would put my two cents worth.

Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Tmoose]

From the description is sounds like it is strictly rpm related, not load or road speed? How about when accelerating a bit in high gear at that speed uphill?

Is this your setup?

https://www.nostalgicairparts.com/ac/ford-flathead-alternator-bracket-124.php

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://www.shop.com/Genuine+Briggs+Stratton+19200+Tachometer-398088266-p+.xhtml

 

[RossABQ]

Thanks for the responses.

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?

 

[Tmoose]

the reed tach would show the frequency of the vibration, and to some extent amplitude.
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

 

[CBuzz]

We use vibration analysis on our equipment all the time (Paper Mill). So there may be some sources for vibration testing in your area other than automotive. Once you know if the vibration is one times crank revolution etc. you can do the math to figure out what part of the motor might be the actual vibration source and not just some multiple of the frequency like the alternator shaking.

 

[RossABQ]

I'll check the flywheel runout and run it with the alternator off the engine this weekend (my daily driver is down). Thanks for the ideas. I'll see if I can scrounge up some vibration measuring equipment.

 

[thruthefence]

Do you know anyone at the around your local airport in the maintenance business? There may be someone there who can hook up a vibration survey on the car. Aircraft Mechanics are usually gearheads, and might enjoy seeing a running L head Ford.

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.used-helicopter.net/wp-content/uploads/2010/12/-19241206534520460.jpg

no financial interest

 

[Warpspeed]

It sounds like some kind of resonance to me, possibly something to do with the sprung centre clutch plate or engine mounts.

 

[RossABQ]

It does this in neutral, clutch in or clutch out. Motor mounts have less than 2,000 miles on them.