Tower bolt threads

[Jono]

Hi,

My Mk1 has been fitted with a later front sub frame using single tower bolts. I have had to replace the sub frame (don't ask ) and obtained a good second hand unit of similar vintage. As far as I can tell the first sub frame came off a circa 1993 version Rover Cooper.

The replacement frame is of unknown vintage but I had thought it to be of a similar age.

My problem is that the tower bolt threads appear to be different. The bolts from the circa 1993 Cooper sub frame have a coarse thread whereas the replacement sub frame appears to have finer female threads (3/4 unf?).

Can anyone provide an account of various thread sizes for the single bolt sub frame? Sadly the garage has wound in the bolts before realising they were, ahem, a bit tight and has buggered the threads on the bolts. I am, perhaps in vain, hoping the sub frame threads will chase out with a tap and be useuable again but I need to try and establish what I am likely to have so that I can obtain new bolts and a suitable tap.

Oddly, the usually suppliers only name one type of tower bolt which is said to suit all post 76 cars. My guess is that the coarse threads are metric, although I have not yet done any measurements.

Cheers

Jon

[guru_1071]

there is only one thread 21a2596 bolt used on the single bolt post 76' frames and its a fine unf thread, if yours looks metric this is normally a sign that the bolt has been crossthreaded and cut its own thread. if this is the case, the frame is scrap.

a tap will be expensive as its huge!

i guess the garage will be replacing the frame for you.......

[Jono]

...but the bolt is definitely a coarse thread where it has not been adulterated, certainly not a UNF.

[guru_1071]

...but the bolt is definitely a coarse thread where it has not been adulterated, certainly not a UNF.

it might be a coarse thread now, but from the factory it would have started life as the standard unf thread.

i have never, ever see a 21a2596 tower bolt with a coarse metric thread, they just dont exist, if they did there would be a seperate part number for them.


what ever has happened to your frame is the result of a crossthreaded tower bolt, the thread damage either 'looks' like a metric coarse thread(which is what happens then the bolts cross thread), or someone has run a metric tap down to try and clear up damage.


either way, once the thread is as goosed as this it will never come back, what your taking out of the threads in the frame cannot be put back onto the threads on the bolt.

[Jono]

...I will say it again - the portion of bolt thread which has not entered the female thread on the sub frame is a coarse thread. I've been tinkering with Minis, Loti, old Fords, Caterhams and many others for over 30 years and so I know a coarse thread when I see one

I hear what you are saying, and I understand it, but what I am actually seeing is different.

[guru_1071]

jono

when the tower bolt is done up tight it will pass right to the end of the thread in the subframe, there is no 'unused' thread left.

if a tower bolt is crossthreaded and wound right in it will damage all the thread, this must be what you are seeing (and a damaged fine unf thread of this size can look metric)

i have seen this loads of times (and done it my self on occasion) people 'wind' the bolts in using a strong arm, it damages the threads very easily and by the time they realise that there is an issue, its too late, they are far more delicate than the size of the bolt would suggest.

[cheleker]

If the bolt is the issue, it's not a Mini bolt.

[Jono]

Guys...but the bolt has not been fully wound in - he realised it was 'tight' and backed it out before it was fully engaged.

The plain shoulder measures 17.5mm across and the undamaged threaded portion 18.0mm o/a. I will post up a photo tomorrow.

Cheers, Jon

[Vegard]

A wet frame would have 5/16unf threads. If these ever stripped, tapping either 10mm or 3/8unc would be a possibility. I did the same, but tapped 3/8unf as per dry frames.

[Spider]

Sounds like someone previously buggered the thread and then re-tapped it, bigger in diameter and with a course thread, or did it by the butcher's method of screwing in the wrong bolt. Another indication is that the bolt doesn't go right through, so it's too short for the job.

Do yourself a favour, scrap the frame, ones in good nick are not hard to come by, or maybe buy a new one that will have perfect alignment, good threads, holes that are not flogged out and not rusted. Mini Spares have them. Beware of those not made in the UK.

[Vegard]

Scrap a frame because of "wrong" threads???
Stupid! Why not fix it?