Roundhouse cylinder cover screws.

[Boustrophedon]

Hi

What size and thread are the current standard Roundhouse cylinder cover screws? The countersunk ones. I thought they were 8BA but they are smaller.

[TonyW]

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[GTB]

What size and thread are the current standard Roundhouse cylinder cover screws? The countersunk ones. I thought they were 8BA but they are smaller.

Both the cheese head screws used for the cylinder end covers and the countersunk screws used on the valve chest covers of current cylinders are 9BA.

If you measure the screws, they are 1.9mm o.d. and 8BA screws are 2.2mm o.d.

The o.d. of an M2 thread will also be 1.9mm when measured, but if you try one they won't fit, due to the different pitch and thread angle.

Regards,
Graeme

[Boustrophedon]

Are you sure? I have just had a 9BA thread gauge on the thread of one of the countersunk screws and it almost fits, but a M2 bolt held alongside the screw fits exactly, so I was tending towards it being M2.

Certainly not 8BA as they are way too big to fit the threads.

All I want to do is make a new valve cover out of brass with nice hex head bolts to hold it down, perhaps with a few dummy ones in between. Now if it is 9BA I can nip down the road to Chronos and buy a bag of reduced head bolts. But if it is M2 then I will have to make the bolts myself.

(Or run a M2 die down a 9BA bolt to "fix" the thread...)

[GTB]

Are you sure? I have just had a 9BA thread gauge on the thread of one of the countersunk screws and it almost fits, but a M2 bolt held alongside the screw fits exactly, so I was tending towards it being M2.

I don't have a BA thread gauge, so can't check the current Roundhouse screws, but the project on my bench at present has Roundhouse cylinders bought from the factory early this year and 9BA screws fitted those OK.

9BA screws will fit into an M2 threaded hole, but not vice-versa, so Roundhouse could have changed to M2 at some point without me noticing. The first time I swapped cylinder end cap screws to hex head type, M2 fasteners definitely didn't fit and I haven't checked since.

An M2 die won't make any change to a 9BA thread, as it will already enter an M2 threaded hole.

Accucraft sell packets of reduced hex head M2 screws, but I don't think they list any that are long enough to be used for RH steam chests.

Regards,
Graeme

[Boustrophedon]

Thanks, I'll buy some reduced head 9BA tomorrow then.