The right degree of slop...

I’ve been wrestling with this P4 chassis for a few weeks, that perhaps sounds over dramatic as it’s not been quite as hard as all that, but combine the fidelity of P4 with plastic centred Gibson wheels and I’ve been nervous…

Perhaps without warrant, as with patience and a realisation that P4 ‘fidelity’ didn’t mean tight coupling rods, especially with the compensation, I’ve achieved a free rolling chassis. I’ve built a number over the years but this was the first that was fully compensated. The vertical movement in every wheel set sounds like a disaster to set up, but actually, with fairly accurately quartered wheels it didn’t take too much adjustment. Mainly patience, and a cool head! Walk away frequently is good advice.

If you’ve got a kit lurking in the drawer then why not get in touch and see if I can breathe some life into it for you. Until next time more soon…


  1. As you've found out, with properly quartered wheels, cranks or counterweights, just a tad bit of play (slop) is all that's required for hinged rods (multiple piece) on a compensated chassis.

    I typically use about 0.001" to 0.0015" (0.0254mm - 0.0381mm) of play between the crankpin OD and the rod ID. This can easily be achieved using chucking reamers in a drill stand (drill press here in the States). The motion tends to be a little tight on the lower end of the tolerance (0.001" of play), requiring some running in with oil (a few minutes in each direction).

    If you don't use self quartering wheels, cranks or counterweights, a good quartering fixture is a MUST for such fine tolerances.

    Great work as always James...

  2. Jeff, I’m always astounded at your precision, I just work to what looks and feels right. In OO/EM that isn’t as much of a problem, in P4 I’ve been worried, but so far so good. Brake gear has gone on today since the video, rods and cylinders next week.


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Thank you for leaving a comment on my blog - I appreciate you taking the time to share your views. James.