Harness shoulder strap bolt

[Simon420R]

Hi everyone, I've recently bought a lovely 420R which I'm slowly fettling and prepping for the spring weather. I've bought a set of TRS 4-point harnesses which I'm going to fit as the Caterham ones have now gone past their sell-by date. I've had to chop off one of the shoulder strap bolts as the track day roll bar was in the way. I've ordered a new 7/16 UNF bolt but have gone for a 1-1/2 length instead of the standard 1-3/4 length to avoid having to remove the roll bar.

What are your thoughts on this? Will safety be seriously compromised? Another option would be to go for a 2-inch bolt and cut it down to size; the standard 1-3/4 bolt was fractionally too long to take out but I could chop a 2-inch to just under 1-3/4 but longer than the 1-1/2 I've bought. 

[ECR]

I'm assuming that this is a through bolt with a nut on it (as it is on my car)? If so and if the bolt is protruding through the nut then you have no problem. (Or have I misunderstood?)

[Roger Ford]

The chassis holes are threaded on newer cars, so no nut. As long as there's a decent length of thread in the boss I wouldn't have thought it was a problem.

[ECR]

In which case a minimum of 7/16 engagement in the bush. MINIMUM

[Simon420R]

I tested the bolt this afternoon and there will be at least 1-1/3 inch (I hate imperial!) into the boss, taking into account the spacer to allow harness rotation. It felt incredibly sturdy.  

[ECR]

Perfect!

[Doc007]

Is this a newish S3? And by "trackday roll bar" do you mean the double diagonal roll bar?

I have a 2015 S3 with the trackday roll cage (the one with the double-D without the side bars to assist entry and with a double-diagonals at the back) and I can't get either lateral harness bolts out because of the diagonals. Does anyone know whether the double-diagonals are the same size/position as on the trackday roll bar?

 

[SamC]

A good rule of thumb is to aim for 1.5 x the bolt diameter of thread engagement into a boss. Beyond that you're not loading the threads sufficiently for them to be adding strength.

By that calc you are more than safe of course.

[Simon420R]

It's a 2018 S3 and, yes, it's the double diagonal bars. One bolt just about came out with around 0.5mm to spare but the other side was about 0.5mm longer and had to be cut off at the head. Clearly the diagonal bars are not symmetrical. 

Luckily the replacement bolt fits perfectly with loads of thread inside the boss, only about 3mm less than the original one. 

Caterham are out of stock of the 7/16 1-3/4 UNF bolts. 

[Doc007]

Thanks, Simon. That's very helpful.
Does anyone know if there is a difference between the geometry of the trackday roll bar and trackday roll cage?

[SLR No.77]

This has been a problem for a lot longer than just recent cars.  The far LH standard size harness bolt in my '99 car with a single diagonal FIA bar could not be removed without unbolting and lifting the rollbar a little, maybe 5-10mm. I fitted a shorter bolt, long enough to be fully inserted in the threaded chassis tube although I can't recall the size.