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TR4 bonnet woes


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my TR4 bonnet was not well secured to the cross member at the front or the webs that come from that

 

it started to crack about 1/3 of the way into the drivers side headlight "eyebrow"

 

had it properly repaired, welded in behind, drilled at end of crack, webs fixed to underside of bonnet etc

 

in no time at all mileage wise its cracking again and migrating inwards, also started to crack on the other side too

 

I see moss have bonnet strengthening pieces

 

can someone please advise

  1. where they are fitted
  2. should the structural cross member be hard up to the underside of the bonnet & sikaflex bonded to the bonnet (or should it float beneath the bonnet like mine?)
  3. should the webs off the cross member be sikaflex bonded also

any other suggestions to remedy this once and for all

 

many thanks

 

Graeme

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Hi Graeme,

 

I have never seen cracking in that area before. The Moss strengthening plates fit further along the bonnet edge, towards the rear, where there is a cross member for the bonnet catch location.

 

From the factory, the front cross member and stiffening bar would of had felt between them and the bonnet skin. I would suggest some closed cell foam would be a good solution.

 

I have been looking for photos but don't appear to have any of that area. Hope the description works.

 

Cheers

Graeme

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Hi Graze,

see attached pic.

 

The front edge and sides of the stiffener are spot welded to the bonnet skin.

There is a length of felt between the stiffener and skin.

At the rear corners there are strengthening plates - Mine are welded tpt he side flange and the cross strengthener.

 

Roger

 

 

 

 

 

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Hi Graze,

 

Good job you are in Australia, what you are describing is a "wobble" board as previously demonstrated by a bearded entertainer now discredited.

 

Think about it, your bonnet is securely fixed either side at the front by the hinges (just like a pair of hands either side of a hardboard sheet) then when the vehicle drives over uneven or bump surfaces the shock is transferred up through the suspension and bodywork fittings through the hinges (just like the hands) and the bonnet subjected to an up and down instantaneous shock with a central area unsupported. The skin flexes in the centre and causes stress in the stiffest area at the edge which is the "eyebrow" pressed area which is where yours is cracking.

As standard the bonnets came with a thick felt pad which was trapped between the skin and the cross bonnet strengthener, (I had to pry my skin and the strengthener apart with my hands to remove the felt, only by 3 or 4mm, and it was originally fixed in place with a sealer on the bonnet side). This served to "dampen" out any vertical shocks and remove the stress from the panel, I fear your strengthening of the eyebrow area without any central supporting felt underneath the cross bonnet pressing just makes the stress worse at the area where the flexing skin meets stiffened eyebrow.

Repair bonnet again and then fit with supporting shock dampening felt pad under the strengthening cross bonnet pressing.

 

Mick Richards

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Hi Alan,

I'm sure you are right. However the strengthening panel would still work.

 

The standard long bulge bonnet TR4 could still deform crack forward of the cross member if the bonnet is dropped.

The short bulge bonnet cracked elsewhere.

 

Roger

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Roger,

 

By 'early' TR4 bonnet, I did not mean, specifically, the short bulge variety.

 

My own TR4s are 'long' bulge but do not have the cone shaped buffer -

just the flat type, similar to TR3As.

 

I was not aware of any specific areas where a short bulge bonnet was

prone to cracking.

 

AlanR

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Hi Alan,

I may be wrong (again) but I thought ST went to the long bulge because of issues with the short.

 

My TR4 also has the flat rubber area but this could still benefit from reinforcement, especially if people slam the bonnet shut,

 

Roger

Edited by RogerH
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Roger,

 

Issues - Yes - but I think the issues were with clearance between the

bonnet and the SU carbs.

 

The factory introduces a device to limit the engine movement on the

early cars.

 

Your photo indicates a later TR4 bonnet that would have cone-shaped

rubber buffers.

 

AlanR

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Hi Alan,

I think we are going round the houses on this.

As I understand it the short bulge had a weak area between the end of the bulge and the rubber buffers and the edges of the bonnet suffered.

The carbs may well have had other problems.

 

But this started with cracking up the front end, which may be due to lack of support.

A similar lack at the back can cause the rear of the bonnet to crack whether it has flat rubbers or coned.

 

Roger

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Ok, gotcha. Just wanted to make sure nothing was getting lost in translation (er...well, you know what I mean).

 

FWIW, I have a short-bulge bonnet (with flat-top rubbers) and the only problem I've come across is the "usual" cracking in the flanges about 8" or so from the rear corners which I think strengthening plates similar to those in Roger's photo will fix. I'm having a whole load of bodywork...ahem..."tidying" done at the moment which will include adding these.

 

Tim

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thanks all

 

Mick's explanation of a wobble board & Roger's photos explain it well

 

my bonnet has a large gap between the bonnet & the cross member so it will need some remedial work to get it to a point where we can fit *& compress some felt or similar between it & the bonnet

 

at least I know what to do now

 

many thanks gents

 

G

Edited by Graze
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Ok, gotcha. Just wanted to make sure nothing was getting lost in translation (er...well, you know what I mean).

 

FWIW, I have a short-bulge bonnet (with flat-top rubbers) and the only problem I've come across is the "usual" cracking in the flanges about 8" or so from the rear corners which I think strengthening plates similar to those in Roger's photo will fix. I'm having a whole load of bodywork...ahem..."tidying" done at the moment which will include adding these.

 

Tim

All the bonnets be they short or long bulge suffered from cracking on the side just in front of the rear re-enforcing which is what the plates are designed to stop. The change from short to long bulge was indeed due to clearance on the tops of the SU`s especially when used in competition hence as Alan says the damper fitment. (Though this was negated later when they went to Strombergs) The change to pointed rear rubbers was done to prevent the bonnet chafing at the sides and also improving the action of the bonnet catch.

Stuart.

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thanks all

 

Mick's explanation of a wobble board & Roger's photos explain it well

 

my bonnet has a large gap between the bonnet & the cross member so it will need some remedial work to get it to a point where we can fit *& compress some felt or similar between it & the bonnet

 

at least I know what to do now

 

many thanks gents

 

G

Hi Rob,

the skin needs supporting. Normally the felt strip does the job.

 

Roger

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