As a new starter in 3mm/14.2 gauge I found your BRM article on Flockburgh particularly informative, especially the warts and all descriptions of the many traps and shortcomings which the 3mm novice will encounter. This has not put me off having a go, on the contrary, I now know what to be on the lookout for. Despite advice from other 3mm members that compensation and gauge widening is not necessary, I am reluctant to believe them having seen how temperamental (to me) some layouts are. I’ve already built some compensated wagons and am now moving on to the first loco, a Finney Smith GER J67 0-6-0.
What I would like to know is how you compensate your loco’s. Do you use the 4mm MJT hornblocks? How do you cut the holes? How do you fix the hornblocks and make the pivot. How do you split the con-rods?
I'll have to agree with you about compensation in 3mm scale 14.2 gauge. While there are people who say it's not necessary, I think they are wrong. Since my layout runs pretty well when I run a locomotive with a flexi chassis and not when it's a solid one, then to me the point is proved. While I can't claim to be the worlds greatest modeller, I am picky about running and locos that stall a lot and fall everywhere have no place in my stock box.
The photo shows the chassis of my 3mm scale Jinty before the wheels etc. were fitted. As you can see there are hornblocks fitted, which you correctly guessed are MJT ones. The only problem with them is that the guides are too deep for the smaller scale chassis. On some locos this can be ignored as the excess is hidden. If it isn't, a piercing saw along the top of the frames sorts the problem out after the chassis has been tested. The same saw is first used to hack square-ish inverted U-shaped holes in the frames themselves centre on the normal axle hole. Many chassis have half-etched guide marks for this.
The pivot hole can be seen just below a frame spacer.- it's drilled after the frames are assembled. Ideally this is done on a pillar drill but accuracy doesn't seem hugely important so careful hand drilling has been OK in the past. It all depends how lazy I'm feeling.
Fitting is fiddly but not too difficult especially if you have a chassis jig. Use the con rods to set the spacing and small clips to hold the guides in place while they are tacked in place. Mind you when you look at my efforts, I'm probably not the best person to listen to. The chassis still works despite it !
One thing to watch - the faces of the hornblocks need to be filed a bit or they are too wide to allow the wheels to fit and get to gauge !
The pivot is a brass rod inside the tube. The tube fits between the frames with the rod extending through the sides. Soldering the rod and letting the tube rotate around it works well, or let the rod rotate in the hole. The beam, another bit of brass rod, is soldered perpendicular to the tube at a later point.
As for the rods, they are supplied ready split in most kits. The lamination's should allow for a joint to be made on the centre axle's crank pin. If this isn't the case, cut the front and back lamination's so the front one on the left overlaps the middle pin, and the rear one on the right does the same. I know it's not a proper forked joint but it's what most people do and works fine.
Hope this helps. Good luck with the kitbuilding. Finescale 3mm isn't the easiest thing in the world but it's worth the effort.
Final hint - if you haven't got it, track down a copy of the aged Iain Rice book on chassis building published by Wild Swan many years ago. It has some excellent diagrams to explain all this stuff.