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MG Midget and Sprite Technical - Panel gap

What is the correct (factory set) panel gap between the rear edge of the bonnet, and the scuttle. Was it ever specified, or just set to look "eye sweet"?
GuyW

I don't know the answer, but I would be amazed if there was a specified gap. If it looks OK then its good enough. I think that was the attitude for all of the British car manufacturers in the sixties and seventies, except for the likes of Rolls-Royce.
Mike Howlett

I did mine by eye, so that it looks right. But I am occasionally getting a light contact between bonnet and scuttle Not every time but annoyingly it's enough to occasionally shed a flake of paint. It needs opening up a tad, or maybe a gnat's whisker. I measured it with a caliper and it is consistent across the width at 0.25" which seems a logical imperial gap.
GuyW

Does it only contact when opening/ closing ?
As it’s intermittent could it be wear in the hinges as there is a lot of weight in those frog bonnets.
richard b

Yes Richard. When it's shut the folded back flange at the back nestles into the V groove in the rubber infill in the "gutter". That flange slopes forwards at about 45 degrees.

By wear in the hinges, I assume you mean in the pivot behind the dash? Either the bolt or the holes in the hinge itslf? What else is there to wear?
GuyW

Yes that is what I was thinking.

Or is there a lot of flexing where the hinges join the bonnet ?
What gaps do others run - being so heavy maybe a bit larger would be a safe guard if it’s flexing.
richard b

There seems to be flexing of the clearance gap between the outer skin, i.e. the bonnet top, and the stiffener panel that runs across the bonnet underneath at the back. I tried putting some semi-rigid packing pieces in there to firm it up but that hasn't solved it.
I could just widen the panel gap, but that is why I was asking what the gap should be. Or alternatively, what gap do others have on Frogeye bonnets please?
GuyW

Since my bonnet hinges in the opposite direction (ala Spitfire) things are a bit different but when I set mine up, I lined the back flange with the center of the groove in the rubber car bonnet seal. Not much help but that's how mine works.
Martin

Guy

Who knows? It’d be great to know how contracts for bodies were put out and how the contracting the companies e.g. Pressed Steel, made and assembled them and then if fronts/bonnets were removed and refitted at the car factory for the production line. And what was the ‘rectification’ area on both assembly lines dealing with?

Reminds me of a pal who worked in an aircraft factory as a production engineer in the 1990s/early 2000s. They had been making the structure of an old design rooted in the 1950s and were switching to a newly designed aircraft. The vices were removed from every ‘fitters’ workbench as components were supposed to be ready to fit, if not they were to redesigned and remade with CAD/CAM rather than be fettled to fit on assembly. The story goes that at the first lunch break the fitters went to B&Q and bought a load of Record bench vices...

Cheers
Mike
M Wood

Martin, thanks. Yes that is how mine is - the flange fits into the centre of that groove. But the flange isn't at 90 degrees to the bonnet top, it is angled at around 45 degrees, so the rear edge of the bonnet actually sits slightly behind the centre of that rubber filler.
GuyW

As one opens the bonnet, the rear edge follows an arc, with the radius being a straight line between the bonnet edge and the centre of the pivot bolt behind the dash. The pivot mounting bolt is fairly high up under the scuttle, but obviously still below the level of the bonnet rear edge.

If the pivot were level with the bonnet, than as one opens, the edge would rise vertically initially before beginning to move rearwards. But in practice the pivot is below that level so as the bonnet opens it is immdiately moving rearwards as well as up. The lower the relative pivot point, the greater the rerwards component of the arc that it follows.

My scuttle, complete with the bonnet hinge pockets and brackets was a new item pre-assembled. I have no way of checking now, but IF the hinge points were innacuratly positioned low, then that could have an impact on this bonnet opening geometry. The simple solution is to move the bonnet forward, but the 1/4" panel gap already seems quite large to me.
GuyW

There is a simpler explanation. Its a 60 year old car. The bonnet has beem dismantled, stripped to bare metal, new wings fitted, sanded and resprayed. It may be simply that the the thing flaps around a bit with the weakened metal. The centre sags as I lift the front up and catches as it passes the edge of the scuttle.

It would explain why if I watch the clearance closely as someone else slowly lifts the bonnet there appears to be ample clearance. If I open the bonnet quickly there is enough flexing for it to flap and kiss the scuttle.
GuyW

Guy

Mine is 4mm at the top but its more than that at the A posts. It clears the scuttle edge ok. There is little flexing of the stiffener which I suspect is your probem. On the other hand the A post gap on mine is 6mm as I could not get the bonnet any lower, indeed I had to lower the rad to get sufficient clearance. I suspect your 'simpler explanation' is the real truth. the cars are 60 plus years old with all sorts of history behind them.
Bob Beaumont

Thanks Bob. 4mm is a lot closer than mine (>6mm)

If the whole rear edge is flexing too much the only fix (other than an even wider gap) that I can think might work would be to add reinforcing to the stiffening panel. Something like a small angle or top hat section welded to the underside of the stiffener, across between the two hinge bases. Painted in, it wouldnt look too out of place.
GuyW

Panel gaps? MGs? - Oh Guy you are a card!....anything less than a inch or so is surely acceptable?
Oggers

Guy,

My bonnet flange is angled but no where near 45 degrees. It's more like 10 to 15. The consistency of BMC manufacturing.
Martin

Mine were set to 3.5mm on the top, down the A pillar and along the sill. Same with the front edge of the door. But I lead loaded and dressed all the shuts when I put on the new skins. By contrast the rear shuts were too tight and needed opening up.
f pollock

My Frog bonnet varies between 1/8" and 3/16" along the scuttle edge. That variation is mostly due to the scuttle edge not being dead straight 1/8 does look too close, I think 3/16 is ok. But maybe we're being too precise about these old cars.
Bill Bretherton

Thanks all, it is helpful to have some other measurements to compare, though they generally are closer than mine, despite mine catching.
I have also checked the rigidity of the bonnet and tbh it is pretty firm and it won't readily deflect at all along the rear edge, despite my earlier comments about it possibly flapping! So I am rather more puzzled now!
GuyW

OK, here's a random contribution that may or may not help...

Are the bonnet hinges stiff, either because they are, themselves, stiff or because the two hinge pins are not aligned with each other? If they are stiff, I can imagine opening the bonnet quickly might cause it to be distorted enough to catch on the rear edge. But if the bonnet is opened slowly, the reluctant hinges manage to move fast enough that the bonnet does not distort and the rear edge does not catch.

Or an I talking b*llocks???
Jonathan Severn

Guy, as Richard asked earlier, does this gap 'closing', only happen when you open the bonnet? Or can you cause it to happen for example, by bouncing the suspension at the front? Or by pushing the bonnet rearwards from the front?

Apart from being much heavier, your frog bonnet opens the same way a mk2 onwards opens --- I think.

I did have a split hinge on mine (mk4), at the u bend section, which allowed the hinge to fold a bit, thus bringing the rear edge of the bonnet closer to the scuttle. My hinges were stiff due to corrosion on the hinge bolts in the scuttle.

But if your hinges are new and solid, how then, when the bonnet is closed, can it possibly move rearwards, -- unless something else is flexing, or the entire hinge is moving rearwards in the scuttle?

Does the rear wing edge section to A post cover, open up a bit, as the bonnet to scuttle section closes?
anamnesis

Bonnet is fine when shut. Doesn't move around and gaps remain constant. Gaps are 1/4" (measured with caliper gauge), across the back and down each A post panel gap. I built these latter gaps up with an extra steel filet to improve the fit when I rebuilt the bonnet. The sill gaps are slightly more along the top of the curved sections, both sides.

The flange along the back of the bonnet is angled forwards sufficient to nestle neatly into the V groove of the rain channel rubber filler when I close the bonnet down. As I lift the bonnet it is occasionally just making a scuff contact between the leading edge of the scuttle and some part of the bonnet rear edge. I am not sure exactly what contacts, either the edge of the bonnet or the flange. Its not always at the same point across the width of the bonnet, but now has 3 chips on the edge between the wipers, and one just outside the n/side wiper.

The bonnet also only just clears the wiper arm spindle boss despite using the correct smaller versions specific to the early cars. I think there is another issue there but will discuss that later with a photo.

I am inclined to try bending the bonnet rear flange slightly further. Where it does contact it is very light and I think I only need to gain 1 or 2mm extra clearance at most. Unfortunately, to get enough clearance to dress that edge I think I will need to remove the bonnet again!
GuyW

If the hinges themselves got over stressed in the past by someone opening too far, or mishapen by some other means the the bonnet will sit fractionally lower and can cause drag like this.
Easy solution if thats the case is to put shims between bonnet and hinges. 2 mm is usually enough.
Try with washers first to see if it gains.
Raising the bonnet 1mm or so above the scuttle might be worth considering, less noticable than damaged paint.
Paul Walbran

Thats an idea. I did spend quite a lot of time getting the bonnet skin level with the scuttle all the way across, using a straight edge. I know raising it was achieved by putting washers under just one end of the bracket - at the rear only, I believe it was. I will have to think through the geometry of it again! As you say, having the bonnet a fraction higher than the scuttle won't show as much ad the paint chips! They are bad enough now that it is going to need a localised respray!
GuyW

This thread was discussed between 25/05/2021 and 27/05/2021

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