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MG MG Y Type - A Lack of Tollerace?

In 1999 I bought new kingpins, trunions, spring pans and wishbones and all the associated bolts washers seals etc.
The hub assemblies were made up at that time and the spring pan / wishbones were also made up but the two assemblies were never connected ....until yesterday...

I tried to connect them 'on the bench' and found that the overall width of the lower trunion with its spacer tube, thrust washers and seal holders was greater than the available gap in the spring pan assembly.

I dismantled and measured everything...the trunion was a few thou too wide, the thrust washers were a few thou too thick but more alarmingly the space between the wishbones was 1/8" too little.

I am aware that the spring pan bolts should be left slack and only tightened once the lower trunion is in place, however, when tightened up the result is to almost 'clamp' the trunion between the wishbones.

Having never gone through this process before I have no idea how 'slack' that pivot should be.

Clearly once totally assembled it is difficult to assess the play in this joint as the action of the spring and damper come into play.

Of all the tricky stages of this rebuild I had anticipated that the assembly of all of these nice new suspension parts was going to be one of the easier jobs........huh!

Please tell me that all is well....I dont think MOSS will appreciate a return of parts after 13 years.
Rob King

I have always just tightened everything up in the lower wishbone, then checked that I can move it in and out sufficient to engage the top trunion before compressing the road spring and fitting it to the top one.

Once all secured, I then tighten everything up.

Do not see that you should have any problems really ... or have I been lucky?

Paul
Paul Barrow

Rob: there is a dimensioned drawing of this assembly in the workshop manual. If you don't have one I can photocopy it for you: this way you can soon see where the problem lies. From memory I think (will confirm) there should be a couple of thou end float between the wishbone end inner faces. This effectively disappears when the whole thing is greased correctly.

If the wishbone outer ends are 1/8" too close, I'd start there. Unless the spring pan is distorted, the two pressed arms must be out of true.

Tim
Tim Griggs

Thanks for your contributions...

Tim, I used the workshop manual when measuring the various components. I think the 'end float' is between the inner face of the thrust washers and the faces of the trunion.

I have discussed the matter with Moss and they are looking into it.

When you think of the number of MGs out there with this front suspension I should be able to find someone who can describe whether this pivot point should be tight when the pan bolts are tightened.
Rob King

In my opinion I cannot see any good reason why the trunion should have any movement after clamping it all up at all. Its job surely is to hold the king pin steady in all 3 dimensions when the car is going down the road while allowing the wheel to turn on its steering axis isnt it? By definition therefore, any excess float would excert wear on the pin ... unless I am mistaken ... surely?

Look forward to a straight forward explanation when you hear back from Moss Rob, and I may well also refer this thread to the Tech guys at Moss USA too.

Paul
Paul Barrow

Hi Paul,

This is the lower pivot point of the suspension which needs to pivot with the extension and compression of the coil spring.
I understand that the max angle of pivot is only from bump stop to bump stop, but my concern is that, if this pivot point is too tight, friction and overheating will cause the joint to fail.

Love to know what the MOSS US boys say.
Rob King

True! I have always been able to use the shims I had to get round that.

Paul
Paul Barrow

Rob.

The critical dimensions are the length of the distance tube and the width of the trunnion.

The distance tube has to be slightly longer than the width of the trunnion.

When the fulcrum bolt is tightened, the distance tube is clamped securely between the thrust washers and the fulcrum pivots on the stationary tube.

The distance tube should be between 0.008" and 0.013" longer than the width of the trunnion to give the appropriate clearance for the trunnion to swivel and grease to lubricate the thrust washer faces.


The gap between the assembled wishbones is not critical. These are jig welded mild steel stampings and were never highly accurate. The arms should be loosely bolted to the pans to keep them in general alignment. Then the fulcrum offered up into position. Once in place the wishbone bolts should be tightened in steps which will keep the assembly in alignment. The fulcrum bolt tightened last.

The fulcrum bolt, thrust washers and distance tube then become a static assembly with the trunnion able to pivot upon it.

I hope this explanation clears things up.

Kelvin.
Kelvin Dodd

Thanks Kelvin for that explanation. I am sure that helps us all understand the geometry must better. Your explanation is very clear and reassuring.
Paul Barrow

Kelvin's description is very clear. What happens if you don't have some end float is that the whole assembly locks up and starts to pivot around the fulcrum bolt in the wishbone ends, elongating them and causing general mayhem. This is to be avoided.... :-)

Look forward to hearing what was causing Rob's problem eventually.
Tim Griggs

Kelvin's explanation is quite correct. See page M21 of the Y type manual. The distance tube is longer than the width of the trunnion. The distance tube is clamped tight between the wishbone sides via the washers and dirt seals (and/or the top shock absorber arms) with clearance (marked "A" total end clearance on illustration M15)(from my 1947 manual)
This allows the trunnion to rotate on the distance tube which acts as as a shaft in the trunnion, which acts as a bearing, hence the trunnion being manufactured from bronze. Thus putting no wear on either the bolt passing through the trunnion of the wishbone (shocker arm) or the wishbone itself. Grease penetrates down through grooves cut within the trunnion to lubricate this joint, but needs regular (1000 mile) attention)

As an aside, Len Shaw, (HMO 909) in his book Rallying in a works M.G. recalls that one of the first components his company (Aero and General) made was the distance tubes in the front suspension of Y types, the company specialised in in high precision turned parts that required hardening.

So, in answer to Rob's original question, Kelvin has got it spot on correct, the distance tube top or bottom should be tight, so just follow his advice and the instructions in the manual. Moss probably supplied everyhing in tolerance.

Dave
D P Jones

This thread was discussed between 23/05/2012 and 31/05/2012

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