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Michael S.:
Yes Jo,
that's actually correct. Ideally, cast iron would be the first choice.
I didn't have any cast iron rods for 3 and 4 millimeters. The screws I use are very soft, and it's possible to drill new holes in both the steel and the cast iron.

Best regards, Michael

Jo:
Morning Michael,

I have never found any small diameter Cast Iron available commercially, 25mm seems to be the smallest you can get. So I normally keep offcuts from larger bits of cast Iron and turn them down to the diameter required to make repair plugs. One source is the round cast Iron plug you get if you use a hole saw to take the centre out when making piston rings.

As you mentioned if the parent and plug material are of different hardness a drill will want to wander into the softer material  :facepalm:

Jo

Jasonb:
The other option is not to start by filling the out of position holes with the inherent risk of subsequent drills and taps wandering or worse.

Start by forming oversize holes in the correct position which if they overlap the old ones can be done by plunging with a suitable milling cutter. You can then make some threaded bushes and bond those into the oversize holes for which my choice would be JBWeld which wil also fill any other gaps or bits of old hole.

The advantage of separate bushes is you can use whatever material is to hand such as steel or brass, you can tap right through so your thread will go deeper. And if you do snap a tap you just throw the bush away rather than have to deal with removing it from the casting.

If you do plug with different materials then plunging with a milling cutter is less likely to wander than a drill though a tap may still want to follow the route of least resistance. But OK for clearance holes.

Zephyrin:
the plan shown comes from Jean-Pierre Duval... a prolific designer of steam engines of all kinds,
too much btr screws to my taste in these drawings, but his Decauville loco is a very popular and renowned drawing.

http://jpduval.free.fr

I remember by the way an old recipe to repair cast iron with a mixture of cast iron fillings and sulfur powder...supposed to give a sealing as hard as cast iron.

Michael S.:
Thanks for the tip, Jason and Zephyrin.

I also have the blueprint for the locomotive.

I would use the cylinder for a steam engine with 4 columns. A simple design is best. The Reeves Trojan would also be possible. It fits the cylinder bore of 25 mm and a stroke of approximately 30 mm. I have to adjust it.
I like the crosshead guide with two rods.
It could go in that direction.

Michael

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