MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Harmonic Balancer

Anyone ever experiment putting a harmonic balancer on a T series engine? My XPEG seems to pick up a vibration around 3000 rpm and disappears around 3600 to 4000 rpm. Not saying my tach is right on the money. PJ
Paul161

Could the vibration be caused by incorrect distributor advance weights? Just a thought. If that is the case, I'll send it off to Jeff this winter for a total rebuild. I static timed the engine at zero and at idle the timing is 2 degrees advanced.
I had a vibration once before in the shift lever when accelerating and retarding the timing a bit seemed to cure it, which gives me the idea the distributor is not advancing at the proper rate. Anyone? PJ
Paul161

Do you have the original steel fan? I replaced mine with a plastic MGB fan and noticed a difference in vibration. Have you had the drive shaft balanced? Just a couple of thoughts.

Ed
ECS Stanfield

There is a fair amount of information about harmonic resonances in the XPAG engine around 4,500 revs, it is said to be destructive through metal fatigue. You may find some information by searching in the archives. However I have never seen a valid solution proposed.
Ian Bowers

I think this was batted around a few years ago so maybe look in the archives. Doesn't sound like timing or dizzy to me. Check to make sure engine/tranny mounts good, and the welds aren't cracked on the tranny mount. If the rear tranny nut is over tightened that will transmit a lot of vibration.
George Butz

George, all mounts are new and the chassis is in excellent condition, the chassis was checked in detail after sand blasting for any welding that might need doing before painting, fortunately none was required. I'm going to have Jeff check the distributor fly weights out, as I don't think it has ever been rebuilt and possibly one or both springs are weak and causing the weights to advance improperly. It should be rebuilt anyway. Thanks for your input! PJ
Paul161

Hi Paul,

Fascinating topic! I don't know much about it but I can make a couple of comments which might be bits in the jigsaw.

Many years ago I heard (from a couple of old T-Type racers) of harmonic balancers being used to reduce the risk of broken crankshafts. I should have pushed for more information back then when the old guys were still with us! I guess vibrations felt by the driver might equate to stresses in the crank ... but I could be wrong.

TCs often get vibrations in the gear-leaver at cruising speeds. I believe this vibration is not as bad in TD and TF because they have the third bearing in the rear gearbox extension. My TC road car suffers from this. I wonder if part of the vibration is caused by the distributor. Not just too much advance or retardation but UNEVEN firing. If a couple of cylinders are advanced and a couple are retarded (due to a slightly bent dizzy shaft or a worn dizzy cam), there might be more power from some cylinders than others, causing a "torsional" vibration?

I am currently rebuilding another dizzy in my s/c TC race car and I am finding that it does NOT fire EXACTLY at the same point for each cylinder! I am using a dial gauge on the shaft and testing with a 12 volt light at crank handle speed but I am not there yet. The old dizzy was perfectly accurate at crank-handle speed but had such worn bushes that at high revs the shaft would have rattled around and caused a lot of spark "scatter". When I get the race car right I must check the issue in the road car!

Of course there are lots of other causes of vibration. A worn clutch pressure plate is one of many a common causes.

Bob Schapel
R L Schapel

Bob, Sounds interesting. The only reason my thoughts are on the distributor malfunctioning is, the vibration is around 3000 to 3600 rpms, not in the 4000 range and it doesn't do it all the time, it comes and goes, actually the vibration quits around 4000 rpms. Like I said before, I don't think it has ever been rebuilt in it's 60+ years, so it's going out for a rebuild and hopefully things will improve, we'll see. Thanks for you input, much appreciated. PJ
Paul161

Paul,
You didn't say,, so Ill ask just to make sure,,, do you get the vibration when the car is stationary??

Steve
Steve Wincze

Steve, never ran it up to 3000 rpms setting still, I'll give it a try tomorrow and see what it feels like. You're thinking drive shaft? PJ
Paul161

Harmonic Balancer - sounds interesting. 'twould be a good time to incorporate the magnets necessary for crankshaft timing and coil packs.

Gord Clark
Rockburn, Qué.
Gord Clark

Messed up the edit, ran out of time. Link is wrong. PJ
Paul161

Ran across an article on the XPAG that comments on a harmonic balancer. "For all practical purposes, it probably doesn't need one." He mentions talking to Vibratech, of Alden, N.Y. Page 4. By Dave Barnett, Vintage MG Club of Southern California. He also says Pete Thelander told him 4,700 RPM is peak harmonic vibration.
http://docplayer.net/20923440-Where-the-rubber-meets-the-road-part-i-the-xpag-engine-by-dave-barnett-vintage-mg-club-of-southern-california.html

If your engine is having odd vibrations at such lower RPM's, any chance your (main) crank bearings are a problem?
D mckellar

Interesting article, but I didn't see any convincing reason why it doesn't need one
Dve H
Dave Hill

Bearings? My engine was totally rebuilt by an ex MG dealer mechanic and is so close in tolerances that it doesn't even drip oil when parked after a long run. I'm slowly coming to the conclusion that it's not RPM related, but speed related. Before the 4.8 rear was replaced with a 4.1, 3000 to 4000 RPMs was not a problem and no vibration, but after installing the 4.1, the speed increased quit a bit and the vibration began. Plus the fact that it's intermittent and not continuous I believe it's a wheel/tire out of balance problem. Before I never ran over 60 mph, now with the 4.1 at 65/70 the off and on vibration begins. The wheels and tires were balanced to "0" using the special wire wheel inserts on a computerized machine. I'm more than likely going to change tires and start over. Tire research will be on the list. This is turning out to be a Winter project, as I don't drive it in cold weather. PJ
Paul161

Drive Shaft?

Turn it 180deg relative to the gearbox and rear axle,and see if that sorts it.
Ian Bowers

Drive shaft is 010 out on the gear box end and 012 out on the rear end with the weights on the opposite side.

Turn it 180 deg relative to the gearbox and rear axle,and see if that sorts it.
I can do that and see if it improves. PJ
Paul161

I believe that the proper set-up for a single driveshaft is to have the transmission yoke and the differential yoke 90 degrees out of phase and the shafts carrying the yokes parallel to each other. This does not affect the balance but does guarantee constant rotational velocity of the 2 shafts. That's what CV joints are all about.
( 2 u-joints, back to back, 90 degrees out of phase).
Please correct me if I remembered this wrong, it's been a while since I set one up.
Chuck
cj schmit

Yes, main bearings.. Just one idea to explain why your apparent harmonic engine vibration is occurring at such low RPM. Main bearings are a known high wear item in the XPAG; with the same reason why as the (high RPM) harmonic vibration. In light of new info on your tight bearings and expert (recent?) rebuild, that can be eliminated. As stated, the "vibration" is only normal at around 4,700 RPM in this engine. i.e. something else is wrong.

It sounds you are on the right track about it being speed related. I imagine you have done a tapered roller bearing modification for the differential pinion carrier, etc? So many things it could be outside the engine that I suppose the elimination process should begin with the obvious suspects, like spinning wheels or driveline. With the 4.1 ratio, if you have stock 19" wheels, you should hit the harmonic engine vibration (in top gear) at right around 100 mph. In stock TC configuration the harmonic would occur at around (theoretical) 80 mph in top gear.
D mckellar

CJ, The drive shaft is the original factory one with new U joints.

DM, It is a TF 1500, not TC.
Paul161

Fluidampr makes viscous crank dampers for rice rockets thru heavy diesel engines. They start around $400, and could likely be adapted or fit to an X engine.
D mckellar

Drive shaft phasing. Just so you know. PJ


Paul161

Paul,
Before you go out and buy new tires, Is there some one in your area who doesn't have this vibration that you can switch tires/rims with?? Just to make sure that it is the tires/rims?
Steve
Steve Wincze

Steve, When I put these new Firestone's on the new wires, they were balanced on a computerized machine and using the spacial wire wheel adapters. They were balanced to "0" specs. When I mounted them on the car, I had a terrible out of balance problem. By moving the wheels around from one position to another I finally got rid of the shake. Problem, the tires are out of round, but below 60 mph there's no vibration, above 60 the vibrations started coming back. 3500 rpms X 4.1 = 63 mph. That's where I got the 3000 to 3500 rpm range from.
Drive shaft is straight, in phase and new U joints, all bearings in the Dave Clark rebuilt rear are new to my knowledge, along with his new hard axles. We have no one around here who can shave and true tires, which would probably be a big help. I'm just going from the front of the car to the back trying to find the source of vibs, only thing left is the tires and wheels. Onward into the darkness! Grin. PJ
Paul161

Paul,
You say the only thing left is tires and wheels,,,,, You didn't mention the crudely casted brake drums. This is what I found;
When I made up my adapter to balance steel wheels/tires "lug centric", I used a spare front brake drum with internals removed. The bare drum was mounted on a conventional spin balancer using the machine's cones, and each wheel/tire was in turn, mounted on this drum, and subsequently balanced.
BUT FIRST, we spun balanced the bare drum, and found that it was out of balance and needed three tape-on weights.
Sooo, the drums are just one more thing that might need attention.

Steve
Steve Wincze

Yes I realize that Steve, possibly that's not the problem here. And, yes they are a crude casting by todays standards, but my drums had very little use over the years compared to most and have no high spots. Three clicks on the adjusters will lock them up. But, if all else fails, another avenue to pursue. PJ
Paul161

This thread was discussed between 09/10/2016 and 14/10/2016

MG TD TF 1500 index

This thread is from the archives. Join the live MG TD TF 1500 BBS now