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MG Midget and Sprite Technical - Bonnet Hinge Bolts

Why are the inner 4 bolts supposed to be longer?
Nick and Cherry Scoop

It's more that the outer 4 are supposed to be shorter, - so they don't put dimples in the bonnet.
GuyW

Thanks Guy. I wonder why the manual didn't say that.
I shall investigate.
Nick and Cherry Scoop

Is that the same for the midget as well? I don't want to make a £545 mistake!!
John Payne

Yes John, l think this would apply to all the bonnets. The clearance above the hinge attachment plate is less on two of the bolts because of the curve of the bonnet and the angle of the plates. It's an easy enough thing to check with a bit of wire or welding rod. I don't off hand recall what length they are but it is worth getting it right!

I am not sure why they even used 2 longer ones as it would be safer to avoid mistakes and have all 4 as short ones. But it is probably easier getting the longer pair started as one is first trying to hold the hinge up against the hinge panel with one hand and it is struggling to move away from its proper position.
GuyW

I get:-
7/8" from face of bonnet plate to bonnet - outers
1 1/8" ditto - inners
5/8" is the thread length of all 8 bolts.

With plain and spring washers, and the thickness of the hinge flange, I estimate that the bolt thread will easily reach to the end of the setnut. Though the first owner had included a thick gasket and lots of gunge between hinge and bonnet plate.
Nick and Cherry Scoop

Mine seems a fraction tighter than that Nick. Shortest clearance a bare 3/4". I would be worried that a 3/4" threaded bolt on that might just nudge the bonnet if there is any movement or vibration in use. 5/8" bolt would be OK though.

Should there be rubber or paper "gaskets" between the hinge and hinge plate though? I added some made from 1/4" conveyor belt material on the other car in an unsuccessful attempt to raise the rear edge of the bonnet relative to the scuttle. That was before I discovered the correct way to do it! But maybe there should be a thin gasket there?
GuyW

Mine all had the shorter bolts and they seemed the original ones (all painted cherry red to match the car) it also had a gasket between the hinge and the bonnet but on one side only. I think these were ways the factory got the panel fit right. My service parts list mentions these as packing part number 14A 4687
Bob Beaumont

Interesting. I looked for those gaskets, but could not find them in the catalogues. I've got plenty of flexible vinyl in slightly the wrong colour, though.

My bolts are also all short, and cherry red.
Nick and Cherry Scoop

I would expect a gasket between the two plates but my impression is that on the frogeye the height of the bonnet is more fixed by the rear lip settling into the V groove of the rubber section sitting in the scuttle drip channel.

The later cars don't have this arrangement. I spent a long time fitting ever increasingly fat spacers between the bonnet hinge plates and the hinges on my Austin Sprite with only a modicum of sucess. Adding the spacers seemed to depress the hinge as much as lift the bonnet. In the end I worked out that putting spacers under just the rear two mounting bolts, and not under the front ones changed the "angle of attack" of the hinge and had much more effect in raising the bonnet relative to the scuttle.

My Firestones arrived yesterday. "Made in Indonesia" but despatched to me from Frankfurt. What will we do without the EU! I guess I will just have to get them direct from Jakarta. Still, they look nice. I don't think I have ever bought 5 new tyres all at the same time!
GuyW

Cheers Guy, that could have been a very annoying mistake! I seem to recall the original bolts had pointed ends which would make matters even worse!!

I’ve just finished painting it this morning and unfortunately only have a small amount of paint left so I can’t afford mistakes.

John Payne

My bonnet is high at the moment. Still, I've only just put a couple of bolts into each hinge, and haven't tried to correct the gaps yet. Have you put the stays on yet, Guy? Or are you hoisting with ropes and bungees as I am?
Nick and Cherry Scoop

Nick, I just prop the bonnet open with an upturned broom under the airbox at the front. It seems pretty stable like that (famous last words!)

When I do get the hinge positions right - which is a way off in the middle distance at the moment, I will drill a 3mm pilot hole somewhere in each bonnet hinge plate and put in a self tapping screw. Then I will have a fix on the position for when I next have to remove and refit it.
GuyW

Good plan. I shall do the same.

My scuttle gaps are 2 and 7 at the moment, so I will aim for matching 4s before I look at anything else. I'm guessing the brand new V-rubber will let the bonnet down a little. By the way, what's the approved method for fixing it?
Nick and Cherry Scoop

I don't know how it was fixed. I suspect originally the rubber was a close enough push fit to stay in place. Mine was sort of stuck down but l think that was because the rubber had decayed rather than a proper adhesive. It broke up and came out in little pieces.

I think the replacement l bought is a little undersized. It's certainly loose in the channel and will need fixing down somehow. Maybe a few short lengths of double sided number plate sticky tape?
GuyW

Yes, mine too. I shall compare it with the original. I've just filled the back axle, so as soon as the bonnet's in some kind of shape I will take her for a run. Only up and down our drive a few times, but with lots of braking.
Nick and Cherry Scoop

Blimey you live and learn. Never knew there were 2 sizes of bolt. After multiple rebuilds I am just using the shortest bolts that work. I can fit the bonnet single handed by suspending it from the roof and lowering the rear end to meet the hinges. For a long time the rear sat proud of the scuttle and I eventually worked out why. I removed the headlamps and their buckets and peered inside with a light. The problem was the hinges, which were distorted. They had stretched with age, so I cut and rewelded them and the bonnet now fits better than when I bought the car 40 years ago.

Les
L B Rose

I checedD my BMC parts book again and it only references one size of set screw for the bonnet hinge. ie all 8 are the same(part no HZS0405) This sort of confirms mine and Nick's findings on our cars. The parts book is dated June 1960. The heritage certificate for my car says it was built in September 1960 so it seems to make sense.
Bob Beaumont

Well, I thought my workshop manual was 1960 or thereabouts, but the other day I noticed that it says "Sprite MkI" on the cover. So maybe a later car had different screw sizes, and it crept into the text.

What's got me foxed is that the bag with the screws in it has a scrap of paper on which I wrote (five years ago) "inner ones longer".
Nick and Cherry Scoop

In that case it was just me adding to the confusion. Given Nick's first statement that there were 2 longer bolts l was pointing out that the shorter bolts were needed at the outer sides to avoid dumpling the bonnet. If all the bolts are the same then so long as they are the originals and all short enough (5/16?) not to mark the bonnet then that must be correct.

I have always fitted bonnets on my own and apart from devising some means of lifting and holding the bonnet in position l find the tricky bit is aligning the hinge with one hand and getting the first 2 hinge bolts on each side to start in their threads. A longer bolt is easier and that may be why Nick has the two lengths of bolt stored from his disassembly.

Alan A says he fits the hinges to the bonnet first and then inserts the pivot bolts behind the dashboard. I tried that too, but found it impossible to get the holes to align with the weight and leverage of the bonnet acting on them.
GuyW

Not quite true, Guy. All the bolts in the bag were exactly the same length. A mystery nobody can help me with, I think.
Nick and Cherry Scoop

Guy, Nick
Moss list the hinge to bonnet bolts as a single item i.e. 8 off 5/8" long bolts.
Bill Bretherton

I thought your original message was specifically BECAUSE you had two longer bolts (per side) in the bag! Where did the idea of the longer bolts arise from then?

I only responded because I choose to use 2 longer and 2 shorter bolts as the longer ones are easier to get started. But in using longer bolts one needs to be aware of the danger of dimpling the bonnet if used in the wrong locations.

I feel this discussion is going round in circles!
GuyW

I follow Andrew A's approach to bonnet hinges. I fiddled around with the hinge adjustment initially to get the best fit I could but thereinafter I just remove the inner pivot bolts. However I have now found I can remove the engine and gearbox without removing the bonnet. Just need to disconnect the stays remove the wipers and lift the bonnet as high as it will go without the hinges touching the scuttle. I used the adjustable removal bracket which attaches to the rocker box. the engine and transmission comes out with just enough room to spare!

Bob Beaumont

Sorry Guy. I did say in post 6 that mine were all short. It was the mystery of my note in the bag, and the manual para that prompted the original question.
Nick and Cherry Scoop

This thread was discussed between 21/07/2018 and 23/07/2018

MG Midget and Sprite Technical index

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