stripped thread

[NigelT]

Seeking words of wisdom on dealing with stripped threads.

When removing my roll bar (to replace it with one that Morgan kindly donated - thanks Morgan, it cleaned up nicely) I discovered one of the two threaded top frame brackets and its bolt (5/16 UNF) to be damaged such that it wouldn't torque down adequately. Tried to insert a new bolt but it immediately started to cross-thread. I plan to get hold of a tap at the weekend in the hope I can clean it out but I think its too far gone.

So, drill out to 3/8, re-tap and fit a 3/8 bolt or fit an insert so a 5/16 bolt can still be used?

There's a guy here in Topanga that's apparently developed an insert system that he says is superior to a helicoil. Apparently affixes through an interference fit.

Any thoughts? Also any general advice on retapping technique would be appreciated having not done this before. I also hear one has to be careful in selecting good quality taps - any recommendations on manufacturers?
Nigel

[rzempel]

So, drill out to 3/8, re-tap and fit a 3/8 bolt or fit an insert so a 5/16 bolt can still be used?

No. Stop. Don't drill a 3/8 hole...you'll not be able to tap it to 3/8 afterwards--the correct hole to drill would be 5/16 for a 3/8-16 or "Q" sized bit for a 3/8-24.

Personally I would (and have many times) use an insert. I prefer Time-Serts for critical applications, but Helicoil works fine for everything else.

Randall

[Doug Liedblad]

Randall is correct, see here for tap drill sizes.

I've used Heli-Coils many times and they will be fine and you won't have different sized bolts in different postions to fool with.

Sears may have the Heli-Coil kit and reasonably good taps if you decide to go 3/8".

The hardware store on Ventura Blvd near Topanga may have them as well.

A Heli Coil kit will have the required tap so all you'll need is the correct drill. It may even come with drill.

If you get them from McMaster Carr they will be good ones but you won't get in time for this weekend. They'll have everything.

Don't buy the taps from Harbor Freight.

Another plan is to come over to my house sometime this weekend. I have the drill and tap to convert to 3/8 but not the heli coil kit. I have a friend that may have it. Will check.

Doug

[Doug Liedblad]

Nigel

I've got the heli coil kit available so you don't need to buy one.

I don't know about your car as I forget what year it is. On mine (2001 build date) there is bolt that comes up from the underside into the center of the roll bar tube. You have to remove the shocks to get to it.

An absolute PITA to get to and mine didn't line up correctly with the hole in the chassis.

Doug

[NigelT]

Thanks guys. Yes, my description was more than a little imprecise. Well, OK, make that totally inaccurate!

Doug, thanks for the very kind offer but I don't think I'm going to be able to make time to do it this weekend. I may come back to you later in the month though if that's OK. I may also check out the insert this guy in Topanga has apparently patented if only because it sounds interesting.

I think the other attachment you refer to may be specific to the FIA bar. This is a standard road bar and is attached solely by those 2 bolts and to the 2 rear brackets. I do plan to fit an FIA bar down the road.

BTW, the steering column vibration has returned under heavy loaded turns so looks like I need to place another washer under the engine mount.

Nigel

[Doug Liedblad]

BTW, the steering column vibration has returned under heavy loaded turns so looks like I need to place another washer under the engine mount.

Nigel

Is that because you are driving it harder as you get used to it?

You are welcome to come by when ready to fix the stripped thread and add another spacer.

I need some notice to get the Helic-Coil kit as it's at a friends. Not far away.

Doug

[rzempel]

Just curious...are you sure that's a 5/16 bolt rather than a metric one? Or, maybe the thread is metric and the bolt is incorrect? I would expect Caterham to use metric--don't they?

[NigelT]

No, it's not a metric chassis. It's a 98.

[Doug Liedblad]

Caterham switched to an all metric chassis just in the last few years.

Mine is 2001 and it's a combination of Metric for the drive train but Imperial for the rest of the chassis.

So far I have not found any Whitworth fastners. But they might be there.

[NigelT]

I was mistaken, the bolts are 5/16 UNC not UNF. But threads on both the bolts are in bad condition. One is just much worse than the other. I'm beginning to wonder if the hole was originally UNF and someone's rammed a UNC in there! Hard to believe that's possible though - I imagine that would make a real mess. Doug, when you get a moment please take a look at yours and check that it's UNC would you? There's very little meat around the hole in the bracket at one point in it's radius (not much more than an 1/8th of an inch) so I fear it may not take a helicoil. I'm going to try to clean up the hole threads with a 5/16 tap and put a fresh, slightly longer bolt in and hope it torques down.

[escondidoron]

Per Doug's earlier post, I would double check that the hole in question is not an 8mm x 1.5. The difference in thread pitch between 8mm x 1.5 and 5/16- 18 is only about 0.0034". And the difference in diameter is about 0.0025", with the 8mm being the slightly bigger of the two. The hex head size is also very close with the 5/16" having a 1/2" head and the 8mm having a 13mm head, the difference being that the 8mm head is maybe 0.011" wider. so with nominal tolerances you can use a 13mm wrench on most 5/16" bolts and, depending on the tolerances, vice versa.

At the end of the day, what this all means is that a 5/16-18 screw will start to thread into an 8mm x 1.5 tapped hole, and vice versa. But after a couple of turns the screw will start to bind up and then strip out the threads on whichever, the hole or the screw, is the softer of the two.

[Doug Liedblad]

Nigel

I've not taken mine apart but am sure they are UNC. I only found one UNF thread on the chassis that I recall and that holds the FIA diagonal roll bar brace to the front mounting point.

If it won't take a Helicoil we can them make them all 3/8-16.

Drill size for 3/8-16 UNC is 5/16.

[NigelT]

According to Caterham UK and USA it's UNF, which explains the mess. Apparently chassis of that vintage were more commonly 1/4 UNF. So, I guess it's time to quit procrastinating and dive on in!

[NigelT]

A 5/16 UNF tap re-cut nicely. Morgan's old bar is now securely in place. If anyone hears of an FIA bar up for sale please let me know. Caterham UK is currently out of stock.

Sorry I missed the SCS run today.
Nigel