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February 20, 2017

Bonnet Latch

The bonnets on TR6 cars were hinged at the front, and so there was a latch mechanism mounted on the cowl to secure the rear in place.  The latch mechanism is pretty typical, where a stud with a cone-shaped end on the bonnet finds its way into a receiving hole on the latch.  On closing the bonnet, the stud forces it's way past a spring loaded catch arm which rebounds behind the cone to capture the stud.  To release the latch, the catch arm is pulled back by way of a Bowden cable operated from the passenger cabin.

My latch assembly was dirty, rusty, and sticky, but intact.

To do a credible refurbish, I felt I had to disassemble the mechanism, which presents a problem since the catch arm is held in place by a staked pivot piece that also holds the sturdy spring.  Removing the pivot isn't actually hard, but it gets sacrificed in the process, so a new one has to be made.

With everything apart, cleaning was much more effective.  An acid bath removed what plating was left.

The catch arm includes the attachment fitting for the actuating cable.  I had an idea for what I think is an improvement for that piece, so I removed it.

Here is the replacement for the catch arm pivot.  Instead of being staked, it will have a threaded stud and be attached with a locknut.  The flats on the top are for a wrench so the nut can be tightened.

And here is the alternate cable attachment fitting.  It's more "in-line" with the cable and I like the looks better.

Now for the real meat of this story.  

Anyone who has spent much time with British cars of this era has certainly experienced the inevitable heartache that comes with the use of so-called Bowden cables.  These are mechanical devices that can transmit a push or a pull over a circuitous path between points A and B.  Many of these cars are litterally infested with them.  Among the several things that can go wrong with Bowden cables is that they can break, which causes the push or pull at one end to fail to appear at the other end.  In the case of a heater control or maybe even a choke this may only be a nuisance, but with the bonnet latch it can constitute a crisis.  Whether it's tinkering or just showing off, LBC bonnets tend to be open a lot, and being locked out of one's engine bay can be enormously stressful for LBC owners.

This is the motivation behind the secondary bonnet latch release.  The idea has been around as long as the cars have.  There are kits commercially available, but many owners just fabricate their own.  Fortunately, adding a backup release is straightforward and simple on the TR6 latch.  

Most of the examples of safety releases I've seen work basically the same way.  The catch arm has an extension on the end opposite the cable fitting.  The extension rides in a slot in the latch body.  Moving the extension to the left will release the bonnet.  In the picture, I've marked the approximate travel needed to release the catch.

A simple right angle lever arrangement, cleverly placed, will move the extension with some mechanical advantage.  Putting a jog in the piece made it easier to connect an actuator to it.  It is fastened to the latch body with a stainless shoulder bolt.

The pivot for the arm is centered in the travel span.

Looks good, but the extension on the catch arm really needed to be a little longer.  Enough preassure on the new lever arm would cause it to ride up on the extension tip and bind.

A blob of weld on the tip ground to shape added about 1/4" to the extension.

Ahhh...  Much better.

With everything now fitting and working well, it all got a nice zinc plate.

On reassembly, I fiddled with the spring way too long before I got smart and enlisted a high-tech aid.

Since the new main cable fitting is lower than the stock one, the little clip that holds the cable sheath has to go on the bottom side of the latch body instead of on the top.

A trial fit on the car showed that I could just drop a rod straight down from the new release lever.  I marked for the hole with a plumb bob, drilled the hole and installed a rubber grommet.

The actuating rod is just a piece of 3/8" stainless with a fork cut on one end and a ball for a handle on the other.  The handle ends up near the heater in the passenger foot well.

There is nothing particularly ingenious or new in this work, but it was an enjoyable project.  It took some time, but the cost was probably under $10.

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