SP 4239, a Mk-3 IMRC AC-10 (Part 1) - Damage Control

In early February 2017, I was able to sit down with a friend and help him with his AC-10 that had a damaged main driver.  Together we worked on repairing it and I took some photos along the way.  A month or so ago, I published my page about the IMRC Cabforwards - 10 Years of Experience.  This is a follow up post to that page.

SP 4239 between runs - Eddie Sims Collection - used with permission.

Mk-3 IMRC AC-10, SP 4239

Driver Damage

Failure of the main crankpin in the driver casting.

The damage took the form of a crack that compromised the grip on the main crankpin of the radial engine.  The entire siderod and main rod assembly including the pin simply fell off the engine.  I happened to have a spare driver and was able to trade it out, replacing the damaged one.

Rebending the Yoke

The yoke is visible between the exhaust steam pipe and the bundle of wires with a 5-10 degree bend in it.

The engine also had a bent radial engine yoke, which is a brass casting.  I was able to bend this back to shape, but something insane had to happen to the engine for this amount of damage to happen to both the driver and equilateral crushing of the yoke from both the right and left sides of the engine.

This was a pain to fix because I had to drop the radial engine from the boiler.  To get to the screw I had to remove the top of the boiler, and the electronic PC board in the smoke box to get to the screw on the sprung stud at the right of the above photo.

Other Damage

To get the drivers out the cover plate had to be removed.  The screws were in so tight that a large amount of force was required to get them out.  I wonder if they're lock-tighted in place or something.  The radial drawbar connection at the main cylinders wasn't able to handle the force required, so it cracked and failed.  I start to wonder at this point if this is part of the damage to the yoke and main crankpin.

Other Modifications

Other modifications to the engine also included rearranging the details in the cab and installing a crew in the engine.  The sliding windows were glued in place to simulate the windows being open and the window sections that held the sliding window parts were removed.



The crew from the 4279 (which is now a scenic display model) were transferred to the 3rd run engine.  Unfortunately the damaged drawbar connection between the radial engine and the main cylinder block was changed between the 1st run and 3rd run engines, so a trading of parts couldn't be made.  So that repair will have to wait until replacement parts can be acquired by the engine's owner.

The old IMRC Mk-1 AC, SP 4279 minus her tender sits in the Bakersfield Roundhouse undergoing repairs.

SP 4279 many years ago became a "parts engine" to use when other parts on IMRC engines failed, broke, or dropped off.  Note the missing front left side rod pin screw and eccentric rod screw have been removed.

Conclusion - Mk-3 IMRC AC-10, SP 4239

The SP 4239 shoving hard at the rear of an SP freight on the Modoc line.  Eddie Sims Collection used with permission.

Hopefully the IMRC SP 4239 will soon enter service and then we can see how the IMRC's Mk-3s perform.

Jason Hill

Related Pages and Links:
IMRC Cabforwards - 10 Years of Experience