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Bearing out of position - how to correct?

Steam engine conn rod dimension error

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Mike Hurley13/08/2021 13:04:39
305 forum posts
87 photos

My 3 x 5 victorian twin steam engine restoration is plodding on, but have come across yet another stumbling block when trying to assemble everything. Due to subtle internal dimesional differences in the two cylinders, (which I should have accounted for but missed!) I found that one conn rod is now a matter of about 4mm too long between bearing centres!

I can't just re-bore the little end in the new position as that would leave no bearing material at the front. I don't think I can just bore a bigger hole for an oversized pice of PB and re-bore as that would leave little steel support top & bottom. One thought was to sweat in a piece of steel to replace the current PB bearing, then re-bore in the new location for a bearing, But am unsure if this would work.

It was a huge job making these conn rods out of solid, so have no desire to repeat the exercise! Any suggestion please?

Dimensions: (mm) overall 250, little end bearing 22 od x 15 bore. 15 deep.

new_conn rods 5.jpg

Ian Parkin13/08/2021 13:09:42
1017 forum posts
236 photos

Cut in half on the rod…. drill a hole down one end ….turn a spigot on the other and sweat together when you have taken out the 4mm

or drill and tap both sides and join with a stud

Edited By Ian Parkin on 13/08/2021 13:10:45

Edited By Ian Parkin on 13/08/2021 13:10:54

JasonB13/08/2021 13:18:17
22588 forum posts
2641 photos
1 articles

How difficult is it to alter the length of one of the piston rods or remake a crosshead?

Howard Lewis13/08/2021 13:58:14
6024 forum posts
14 photos

If you don't want to cut / weld the rod, could you reclaim the situation by making eccentric bushes at both ends, ie 2 mm shift at each end to get the required 4 mm change?


old mart13/08/2021 14:56:35
3721 forum posts
233 photos

Howard's suggestion of sharing the rework at both ends would be my choice if there was enough material.

JasonB13/08/2021 15:15:25
22588 forum posts
2641 photos
1 articles

You say the problem is due to differences in the cylinders.

If you start playing with conrod length to try and correct it you will find that the cross head and slippers will be out of position and as was shown on a thread earlier this year an 1/8" change to the conrod length had the crosshead hitting the guide bar supports on another engine, hence why I asked about piston rod or cross head alterations.

Mike Hurley13/08/2021 16:18:22
305 forum posts
87 photos

Thanks for the feedback guys.

JasonB - The piston rod and part of the crosshead are a one-piece forging. I have been trying to retain as much of the original parts as possible, and I've already had to modify the ends where they attach to the pistons as those were completely ruined and had to be re-made, but due to severe wear and missing material I had to modify the connections into the piston which now leaves no room for further change. I do take on board your comments about positioning though.

Ian, Had considered this, but my concern here would be re-aligning things after joining as there wouldn't be any room for error as both ends are abviously currently in line, and the tinyest error would stop the smooth running of the mechanism.

Howard / mart. : I'll have a look at how feasible that idea is. Cheers

regards to all.

Paul Kemp14/08/2021 01:58:42
710 forum posts
27 photos

I can't remember the make up of your cylinders but can you insert a spacer between the cylinder and trunk guide / support?


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