Seemed like a good idea to start a thread here rather than scatter stuff all over the place. First some observations on Slaters driving wheels in S7 that I bought for an 8F project. When I received them I put a couple of wheels on an axle in the motor/gearbox and ran them out of pure curiosity. There was quite visible runout on the first set, and subsequently on all the others too. Put all the wheels and axles in a collet chuck in the lathe and clocked the back of the rim to see by how much they were out of true. They varied from 2 to 15 thou TIR. I was surprised how small a runout is noticeable. The eyeball and the brain are rascals! 15 thou looks terrible, and even 2 thou is noticeable without looking too hard. Here's a question for S7 folk - what is the allowable variation in the B-to-B dimension? Apart from looking goofy, at what point does run out start to have an effect on clearance at crossings and check rails? Some advice elsewhere on WT suggested flattening the back of the wheels on fine emery until rims, spokes and axle inserts all end up on the same plane. Also checked and removed tiny machining burrs on the axle ends. It helps as well to have the axle ends and the back of the axle inserts absolutely free of dirt and debris before assembly. All seemed like sensible advice and it did improve matters a bit, but still noticeable runout on all the wheels. Thinking about how the wheels are manufactured gives us a clue. The tyre and the inserts are put in an injection moulding tool. The tool is closed and plastic is injected to form the boss, spokes and rim. After a cooling period the tool is opened and the moulded wheel is ejected. The geometrical relationship between axle insert and tyre depends on the accuracy of the tool and the dimensional stability of the injected plastic. I'm fortunate to have access at work to a guy who has spent his whole professional life in the injection moulding business. He helps us design moulded parts and helps our suppliers design and commission tooling to make those parts. Fair to say he knows his stuff. I took a wheel and axle to work and explained the issue. His opinion was that it would not be possible to end up with perfectly true wheels made this way. The reason is that the plastic (a glass filled nylon in this case?) contracts on cooling. On these 8F driving wheels the crank boss is quite large relative to the diameter on the wheel and the distribution of plastic around the axle insert is not symmetrical. Consequently when it cools it shrinks unevenly and pulls the tyre slightly out of true relative to the axle insert. If you look closely it is possible to see some sink marks in the boss between the inserts where the larger mass of plastic has cooled. There must also be some clearance in the tool to accept the inserts. So...a perfectly true Slaters driving wheel and axle is improbable. Spoked wheels without crankpin bosses ought to shrink more evenly, so should run truer, but I've not measured any yet. Not knocking Slaters though. There are limitations the process, there's a price we're prepared to pay for wheels and there's a need to see a return on tooling investment. I guess they suit most of us most of the time and in 7mm we'd be much the poorer without Slaters wheels. Oh, and quartering's done for us as well! Much to be grateful for. Can they be further improved? The answer is 'yes, a bit'. Some more experimenting and measuring in the lathe showed that: Taking a wheel off an axle and replacing it in the same position can cause the TIR to change slightly by about 2-3 thou, so there's a bit of unrepeatability in the assembly process. Taking a wheel off an axle and clocking it round one flat causes the runout to stay in the same position relative the the wheel, so most of the runout is in the wheel and not any inaccuracy in the axle end (caveat, 1 above). Changing wheels onto different axle ends causes some variation of runout but not much more than 1. above. So little prospect of matching up wheels and axles to minimise runout. On some axles, on close inspection, the back of the wheel insert was not pulled tight up to the shoulder on the axle when the screw was tightened. Wheels fitted to these axle ends always had a degree of free play. Easy to miss this. This turned out to have been because the screw was bottoming in the axle thread before pulling the wheel up tight. Filing about 0.5mm off the end of the screws fixed this. Still left with more runout than I'd like on most of the wheels. In the end I resolved to true them up the best I could and see how they looked when installed in a running chassis. I decided to keep each wheel associated with the same axle end and the same clock position on that axle end throughout assembly. To do that I cut a tiny notch in the square of one end of each axle with a piercing saw blade. That indicates left hand side and top centre position of the crank. The position of the right hand wheel on the axle is then determined by the right hand lead (in the case of an 8F - and most other 2 cylinder locomotives, but not always). Each axle then gets a patch of coloured paint on each of its wheels. Leading axle is purple, then green, orange, magenta for 2, 3, 4 respectively. The axles will always go into the chassis in that order to minimise variation when setting up the rods (if there is some variation in crank throw, bearing in mind plastic shrinkage, it's as well to have them in the same place every time). Also one wheel can be fixed to an axle now and need never come off during assembly, so once made to run true it won't be disturbed again (see point 1 above). And to keep that lot straight in my head for the duration of the project I made a wooden holder to keep them in order on the bench. To true them up I put them back in the lathe with the indicator and with the wheel firmly screwed onto it's respective axle end I simply twisted the wheel on the end of the axle to minimise the indicator reading. Impossible to do by eyeball, but easy enough with an indicator. Brutal, and I had wondered if I'd loosen the insert or damage the axle end, but apparently not. Warranty voided and then some if I break one like this. In the end I have them all less than 3 thou TIR. I have to accept that there may be some variation on the 'unfixed' wheels when I assemble them. That seems to be the best I can achieve, and probably the best the manufacturing process can achieve. There's another experiment I'll do when I have the time, and that is to try and bore the axle insert out in the lathe to a 3/16" press fit. It ought to be possible that way to get perfectly true wheels. Well, as true as the shonky lathe permits, but there are ways to minimise the shonk.