Advice needed : 9 BA stripped hole repair in cast iron (model steam engine)

[vaxxine]

First post here and hope someone can help.

This a 9 BA cap screw that threads into a cast iron pedestal. The screw is good. The threads in the hole are not. 9BA is only approx 0.075 OD so pretty small stuff. Looking for advice on repairing this. This is one of 2 screws holding down the inboard bearing on a Cirrus Beam engine and it also happens to be on the crank side - which makes me wonder if 9BA is too simply small for the applied stress.

In the pic, I have a needle pointing at the cap screw in question

Thanks in advance....Phil

 

[Cogsy]

I'm a metric guy generally so that's where I'd head. 0.075 is approx. 1.9mm so I'd enlarge it to 2.5mm and run a M3x0.5 thread, possibly with the head turned down somewhat if there's not the room. M3 should give you a strength increase as well. Otherwise, 2-56 is close, with a recommended 0.07 hole and OD of .086 but you're going to get somewhat less thread % than usual.

 

[deverett]

Welcome to the forum, Phil. How about an introduction in the appropriate place?

Now to your problem. 9 BA is a strange size for a cap screw, but if that is what you've got... Your model being a period engine, it should not have cap screws anywhere!

I would flush the hole with thinners then fill the hole with JB Weld and drill/tap for 7 BA (99 thou compared to your present 75 thou). There is plenty of metal below so if you are worried about strength, go deeper than original.

Use the correct fixing - studs and nuts. Don't spoil the ship for a hap'worth of tar. To make your model look better, replace the other 9 BA cap screws with 7 BA studs and nuts.

Dave
The Emerald Isle

 

[Blogwitch]

If you don't have the means to open things up to another size, it looks like you have plenty of meat that the screw is going into.
Maybe the easiest way for you is just drill and tap down further and fit a longer screw and disregard the stripped out part.

John

 

[vaxxine]

Al - Dave - John : thank you very much. Your suggestions confirm what I have been envisioning might be required. Being in Canada, BA hardware is not likely available anywhere although it may well be in the US (I'm only 20 minutes from the NY State border). And of course it's readily available via mail order. But if I drill and go to a larger size, I may just go with standard or metric. I like the studs and nuts approach Dave suggested. The only way I know this is 9BA is that Cotswold confirmed they use BA fasteners, and I measured the OD of the threads with a micrometer and compared to a table. The JB Weld (or Loctite Form A Thread) might be worth a try as an experiment but the thread is so fine that I don't know how well suited that approach would be.

This is not a vintage piece. Its a complete "plant" from Cotswold Heritage. It would have been made sometime within the last few years. I'm the 3rd owner so I don't know when it was made or it's age. Studying pics of this piece on the Cotswold web site and in their catalogue, it appears that they've alternated between these tiny Allen cap screws and what look like larger hex head screws. They were not forth coming at all with repair suggestions or any details about the mounting hardware at this location on the assembly.

 

[packrat]

Do you have a set of # drills.? You can get british association thread taps on the net.

 

[vaxxine]

Do you have a set of # drills.? You can get british association thread taps on the net.

No I don't. I was researching that last night and learned that a set of the proper drills and tap/die set for BA would be required if I stay with BA for the repair.

 

[Buchanan]

You could drill right through the casting and use a long screw and put a nut on the bottom with a lock nut as well. Then you don't have to tap anything.
Buchanan

 

[MachineTom]

Let your fingers do the walking. British Fasteners is in NY, yes that easy. I see a BA tap 0-9 sizes. Need to check for the bolts that are available. I have a couple of Witworth bolted machines, they have come through for me so far.
Nice folks.

 

[vaxxine]

You could drill right through the casting and use a long screw and put a nut on the bottom with a lock nut as well. Then you don't have to tap anything.
Buchanan

Yes, that has occured to me too. I'll see what I have to work with once I take it apart.

 

[Jasonb]

It seems quite strange that Cotswold would use 9BA cap heads, 9BA is a non prefered size and would be almost impossible to source cap heads in that size here for a number of years.

Any chance it is M2 as that is theoretically only a thou or two larger and commercial fixings are often a bit under size.

Either drill and tap deeper with M2 which will be easier for you to get hoild of or drill out to 2.1mm (may be loose enough in teh stripped hole) and tap M2.5

#2-56 would be another alternative to M2.5

 

[vaxxine]

It seems quite strange that Cotswold would use 9BA cap heads, 9BA is a non prefered size and would be almost impossible to source cap heads in that size here for a number of years.

Any chance it is M2 as that is theoretically only a thou or two larger and commercial fixings are often a bit under size.

Either drill and tap deeper with M2 which will be easier for you to get hoild of or drill out to 2.1mm (may be loose enough in teh stripped hole) and tap M2.5

#2-56 would be another alternative to M2.5

Jason - thanks for that. I've been pondering metric since it is easier to source. Ya know, the 9BA is my own deduction based on Cotswold saying they used all BA fasteners, plus my measurement of the OD of the thread. I could not get the Cotswold guy to divulge much of anything for some reason; it was like pulling teeth. Anyway metric thread gauges are readily available here so I'll recheck.

I have to admit that I feel a little humbled. I've joined a forum with members who have machined and built all sorts of things for years. And here I am figuring out a simple repair for a stripped thread. Jeez.

 

[mcostello]

Some things are not simple if You have never done them.

 

[stragenmitsuko]

A 9BA has an od of 1.9mm and a pitch of 0.39mm
M2 has an od of 2mm and a pitch of 0.4mm .

Almost impossible to tell the difference if you'de ask me .

There's also M2.5 available with a pitch of 0.45mm , wich micht be a good
way to solve your problem .
Drill the existing holes with a 2mm ( 2.05 ) drill and you're good to tap .
Drilling with a 2mm drill will only remove the existing damaged threads .

Pat