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MG MGA - Engine Vibration - Buzz

Hello All,

Some months back I started a thread titled "engine Vibration" After some suggestions I thought the problem had been solved however the vibration remains. Since the last post I have removed the engine and replaced the rear mount (gearbox) also had the drive shaft refurbished / balanced (new spline and u-joints). At the same time I replaced the rear gear box bearing and seal. The spline was replaced due to some lateral wear and the u-joints as milage on the parts was about 50,000 so more as a precaution.

Originally I had a 1622 engine fitted for 10+ years and 50,000 miles (no problems) then earlier this year decided to refit the correct 1600 block.

The engine was bored 0.020, with 0.010 undersize mains and big ends, new pistons / rings compression about 8.5 :1. Slightly modified cam (higher lift and duration) Relief "eyebrows" cut into cylinder block. 18V type followers and pushrods fitted. MGB type harmonic balancer fitted. The head "O" type from Austin 1800 Mk2 has been refurbished since the last post after I discovered low compression in cylinders 3 & 4, which turned out to be recessed inlet valves. In addition I have had the distributor re-curved and fitted a petronix conversion.

Overall the vibration seems a bit less but remains. Starts at just over 1000 RPM gets a bit worse around 2500RPM then smooths out around 3000 RPM (At this stage I have not exceeded 4000 RPM)

A POSSIBLE CAUSE(S) 1 & 2

(1) Today I was reading through Barney's site and came across some information about Flywheel to pressure plate locating dowels. The site mentions a difference of 0.003 between the diameter of the locating pins and the location hole in the pressure plate. If my memory is correct There is considerably more clearance than 0.003, and I'm certain that I could move the pressure plate much more than 0.003 when located on the dowels. Again from memory the dowel pins are worn / corroded and less than 0.025.

Both the pressure plate and flywheel (Lightened about 8 lbs) were balanced separately and as one. So maybe when tightening down the pressure plate it is now slightly off centre.

I don't want to remove the engine again if it is not necessary, so seeking some thoughts about possible cause above.

(2) I noticed when looking at SPL that there should be packing between the radiator and the body attachment points. My car does not have any packing the radiator being bolted direct onto the car.

Is it possible that this could set up a vibration?

Any thoughts appreciated.

Russell
R J Goebel

well, the flywheel and clutch are common items with the two motors.
Art Pearse

My bet is clutch cover/pressure plate assembly. These are usually good (occasionally bad). If the alignment pins are worn or corroded, the pressure plate could be sightly off center. This is a fairly heavy part, so a slight misalignment can make for significant imbalance.
Barney Gaylord

Hi

Art: I forgot to mention that the modified flywheel fitted to the 1600 engine was a spare one, so not previously fitted to the 1622 motor.

Barney: I was trying to find a reason not to pull the motor again, now it looks like the motor will be out again this weekend.

Will post the result when it's all back together.

best wishes

Russell
R J Goebel

If your going to go that far why not use an early 1800 flywheel and clutch basket. You will need the clutch lever and mounting plate to the transmission from an early B also. That assembly is how my car is set up.

I'm sure it weighs less than a stock MGAs.

If you can get your hands on those parts I think you could call it an upgrade and solve your issue.

Just a thought.
Steven Devine

I have fitted a rebuilt/balanced/lightened flywheel 3 bearing 1800 engine to my A this year and I have similar problem of a moderate vibration in the same rev range. I have tried all sorts of things with no success. Is there anything else I should try before pulling the engine and trying to reposition the pressure plate??? Your hoping I can avoid pulling the engine!

Paul

Paul Dean

Have you set the fuel mixture correctly, it may just be running too lean in that range and the vibration is caused by pre detonation. If it is, it will oil your new engine quickly, so try richening the mixture a few flats and see if that helps before pulling everything again.
dominic clancy

I have to go with Barney on this one. At high revs balance is critical. Assuming you have dowel pin slop, if the flywheel and pressure plate where balanced separately, it is very likely there must have been some imbalance once assembled which would require re-balancing. Can you find out if this was so? You really have to ensure those dowels are a perfect fit.
Another possibility here I guess - with the lightened flywheel and what may now be an eccentric pressure plate any imbalance from this could be magnified when compared to the higher mass of a standard weight flywheel.
If you do take it down again!!! - do check the flywheel face is running true as you turn the crank. If not that could cause vibration too. I found my crankshaft's flange had got burred around the edge during its stay at the grinders causing the flywheel face to run out on trial assembly. You just need a file to remove the burrs and patience until it comes right.
The radiator mounting won't make a scrap of difference to any vibration - the rubber was probably just to stop any fretting and excess strain on the radiator.
Pete
PeteT

Hi All,

The engine is out and on the garage floor and I have made the following observations, Please refer to attached image:

1. The three springs (numbered 3) holding the plate realise lever (numbered 8) is loose and even though the springs are in place these are not under any load. The "plate" can move freely sideways / up and down by approx. 0.10. I noticed when the engine was on the garage floor that the plate was completely off centre - although this might have happened when separating the engine and gearbox.

I have a second pressure plate and checked the release lever plate and discovered it is held firmly in place under spring pressure.

2. The two retaining pegs were not as worn as first thought - measures 0.245 - 0.244 (these should be 0.247) Consequently there is some movement when the pressure plate is placed on the flywheel (without clutch plate). Given above measurements I would estimate the movement at 0.010. The locating hole in the pressure plate is exactly 0.250.

I am thinking that the combination of 1 & 2 would be enough to through the balance out. So my plan is to get new retaining pegs fitted, swap the springs and plate from the other pressure plate and get the balance checked of the flywheel and pressure plate as well as the combined balance.

Thoughts please am I overdoing this?

Steve:

Wanted to avoid purchasing another pressure plate (MGB) and associated parts. This might change if above does not work out.

Paul:

Have you checked for a vacuum leak, between the carby and distributor.

Dominic:

Had the plugs burning spot on with a tan colouring, so confident the fuel is ok.

Pete;

The flywheel and pressure plate were balanced separately and as one, so should be ok. As a matter of interest I weighed the Flywheel and pressure plate, these were 20.4 pounds and 10.2 pounds. So it would seem that the pressure plate now being 50% of the flywheel weight any imperfections would be magnified. Checked the flywheel runout and measured under 0.002, I read a spec somewhere that suggests a figure under 0.003 is acceptable.

Any more thoughts?

Russell



R J Goebel

Russell - I can't find any mention of crankshaft balancing, do we assume you are using your original crank out of the 1622 case? In which case the balance here should be OK?
I also wonder what the 'MGB type harmonic balancer' is for on your MGA engine. Not familiar with this thing, could that cause any vibration?
You are certainly having a good go - good luck.
Pete
PeteT

The harmonic damper crank pulley is a good addition, as the description suggests it stops the harmonic vibrations in the crank, and was added by BMC to later B-series engined cars to reduce crank breakage.

Sounds to me like you have a faulty pressure plate, as those springs should always be under tension and the plate solid and impose to move by hand. I would suggest you should just renew the whole thing rather than try to rebuild it. The clearances on the pegs are not going to matter that much.

dominic clancy

Pete,

The Crank is a 1600 undersize by 0.010, mains and big ends and rebalanced.

Dominic,

I used the naming convention from the SPL for the parts described. The "plate" shown as number 8 is the part in contact with the "carbon ring" not the plate in contact with the actual clutch plate. Better described as a "release plate"

The whole pressure plate was refurbished, and only has 1,000 miles service, since initial installation about 3 months ago. Unfortunately the supplier I bought it from went broke and the business is under new management, so not much hope of getting it sorted by the supplier.

It should only be a matter of swapping out the 3 small retaining springs. This will then centre the "release plate"

If the "release plate" has been able to move freely under hand pressure then hopefully this will be the cause of the vibration. I don't want to reinstall the motor until the problem has been identified and sorted.

I don't have any engineering experience, however it seems to me that if this "release plate" is free to move so far from the "centre of gravity" then this could cause the vibration.

Russell
R J Goebel

Are the pistons and rods equalized?
Art Pearse

Yes, Rods and pistons were balanced.

Russell
R J Goebel

You mean the hexagonal shaped plate that contacts the levers, that I had already understood.

But it should be rock solid under tension from the spring levers, not rattling around loose, as you can see from your other pressure plate.

I would still just replace it with a new one, it's such a big job for most people to pull and install the engine for most things like this.
dominic clancy

I have found the MGB clutch assembly to be a much more solid item and does not suffer from the judder I frequently experienced with the MGA assembly. I have it in my 1800 engine. Would it fit the 1622? If so, may be worth considering.

Steve
Steve Gyles

Hi

Dominic, I was thinking that I might have been describing the wrong part.

Steve, I read the info on Barney's site regards installing an MGB clutch into a high starter position 1600, which can result in some interference issues, not a major issue but can require some "fettling" of the Bell housing. I spoke with an MG bloke today (he helped with the pressure plate) who has converted several high starter 1600's to MGB clutches without any interference issues...so maybe it is possible on my car.

Today I switched over the release plate from the old Pressure Plate it is now rock solid. The loose release plate was marked "Repco" so at some time over the last 50+ years had been replaced by an after market part. Looking at the original Borg & Beck part it seems to be of superior design probably 2x thicker and recessed on the back surface so that it sits at the correct height. The design would also aid self centring. I measured the finish height there was only a difference of 0.010 between the old and new. The end result being there is considerably more tension on the retaining springs. So hopefully this will solve the problem & when the motor is re-installed it remains in the car for another 10 years (I hope)

Tomorrow off the engineering works to check the balance.

Will give an update on the result when the engine is reinstalled.


Russell

R J Goebel

I too use the MGB pressure plate, but you need the front gearbox cover and release bearing fork as well.

I had major interference problems with the Hi-Gear conversion, which was down to a production change that had not been approved first with Peter Gamble. But he was very nice and constructive in finding a solution (in comparison to certain other vendors). It took a few attempts to grind away enough bellhousing but since then it has been great.
dominic clancy

Dear All,

Update from the Engineering workshop.

Earlier today I went to the engineering shop that originally did all of the machine work on my 1600 block with the pressure plate and flywheel. The owner seemed concerned that he may have got the balance wrong so said he would check immediately and let me know. After two progress updates he disassembled the Pressure Plate and discovered that the previous "refurbishment" was not much more than a coat of black paint. The back of the pressure plate (the actual plate in contact with the clutch disk) was so badly worn that the "plate" was off centre and causing the imbalance. Due to wear the plate movement happened as soon as the pressure plate went into service.

Disappointing, regards the refurbishment and in hind site I should have gone for a new part at the time. One positive the engineering workshop did not bill me for the work today.....so one for the good guys.

As mentioned in my earlier post the shop from where I purchased the pressure plate is now under new management...so no chance of any refund from that source.

The good news, by next week I should have the car back on the road .....vibration free.

Russell
R J Goebel

Well...I'm glad you got to the bottom of it and let us know the solution. Good job and good luck with it! :-)
Steven Devine

This thread was discussed between 14/08/2014 and 20/08/2014

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