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Kitchen table P4 conversion of RTR loco wheels

Posted: Sun Feb 09, 2020 5:37 pm
by Simon_S
There has been much discussion about converting RTR loco wheels to P4 by machining the original tyres on a lathe and press fitting P4 tyres. I thought I would see if this could be done with just hand tools. My guinea pig is a Heljan class 07 0-6-0 diesel shunter with 3’ 6” diameter wheels of a quite distinctive design.
1. Check that the internal diameter of the P4 tyres is larger than the RTR wheel centre. In this case, I used Alan Gibson 14mm coach wheel tyres with an internal diameter of 13mm and the plastic Heljan wheel centre is 12mm diameter so there will be some of the RTR tyre left when it is turned down to accept the P4 tyre.
2. Prepare the tyres – the AG tyres have a narrow lip of 12mm internal diameter on the front face, I removed it with a bullet shaped grinding tool in a mini-drill. This was easily done in just a few moments. The photo below shows an unmodified tyre on the left and a modified one on the right, along with the tool used. It was clear to see when the lip was removed as the chemical blacking on the rest of the bore started to be ground off.

3. Remove the RTR wheels from the chassis and one wheel from each axle.
4. Reduce the outside diameter of the RTR wheel. Insert the axle and remaining wheel in the chuck of a mini-drill on a high speed setting, hold the drill horizontal and press the flange against a largish flat file held on a bench top. Using a high speed prevents any imbalance in the wheel from causing more material to be removed from one side of the wheel than the other due to the eccentric rotation.
5. Keep measuring the OD of the remaining wheel centre, it will cut quite quickly.
6. When the OD is down to 0.1mm larger than the ID of your tyre, switch to using an oilstone or wetted wet and dry paper to get a finer finish and accurate final size.
7. Stop when you reach 0.03mm over the tyre ID which will provide a light interference fit. I used a cheap digital calliper for the measurement. At this point, the plastic wheel centre was just starting to break through the original tyre.

8. Apply Loctite to the wheel centre (I used Loctite 638 high strength retaining compound) and press it into the tyre between two M10 washers in a vice.

9. Here’s the finished wheel:


10. Fit the wheels to slightly longer axles, quarter them and reassemble the chassis. I have also spaced the cosmetic sideframes out by 1mm each side and adjusted the pick-ups to suit.

And here’s the finished loco doing a lap of honour on Calcutta Sidings

Re: Kitchen table P4 conversion of RTR loco wheels

Posted: Sun Feb 09, 2020 11:58 pm
by Horsetan
Highly interesting. :thumb

Now that those Ultrascale tyres are available separately, it may be that your method also lends itself to converting HO RTR profiles to P87.