Piston Bottoming out

Hi,
I am trying to complete a build on a Burrell 4'' DCC which I started 4 years ago. I have did encounter a few issues such as the weighshaft was too high but STW very kindly sorted that out for me straight away and took 2.5mm off the base, so that is now perfect.
I now have a problem with the HP bottoming out on the rear cylinder cover plate at BTC, I am on the understanding that the conrods should be screwed tight into the crosshead and then locked up by the two small grub screws in the crosshead. This is not just a few thou out, I had to undo the conrod from the crosshead by 3mm, so I will need to put a washer/spacer between the conrod and the crosshead to solve this issue. I then had a look at the LP side and I found this was also bottoming out but only just, in fact I was still able to rotate the crank but this also needs a small washer/spacer just to give it some clearance. This particular kit 20 came to me in May 2016, so I have had a little while. I did see that member High Pressure mentioned a long time ago about an issue of the pistons bottoming out but I wondered if any body else had experienced the same issues. I measured my conrods from the underneath of the piston to the shoulder at the start of the thread and they were both the same , just short of 150mm. With the spacer in place if I move the HP to TDC the top of the piston is just level with the bottom of the slot in the cylinder bore , which I think is perfect.
Any comments would be much appreciated .
Charles

Hi Charles,

Was trying to remember the time I installed piston and crankshaft. I have to confess I just put it all together as I was convinced with former assemblies that there should be no mistake other than what I did. It all fitted together first try.

Coming back to your question: as both position of cylinder and cankshaft are set by their mounting holes I would assume there to be only a very small tolerance or possibility of (mis-) alignment.

I think to remember HighPressure had one of the „one-piece“ designs of cylinder block whereas you and me should be on the newer „two-piece“ designs with separate cover plate on the rear of the cylinder block.  So I would suggest to get into contact with Will with the dimensions you have. I trust he can have a look at the drawings and double-check if there is a wrong part somewhere.

Sorry to be of no more help.

Cheers

Elwood

Hi

The original production run had issue of bottoming out on both pistons.
This was due to the rod glands being too long and protruding too far in to the cylinders.

Undo the nuts for the packing glands, with the piston rods fully screwed in to the cross heads. Observe if the gland packing is pushed out of the block or not.
This will assist with the answer.

This is only valid on the one piece blocks. But worth a check on the two part as well.

Thank you Elwood and Lynn,
I am really not sure why mine has this issue, because I assume my block is the the two piece block ie. it has a plate screwed onto to the underneath.
I have checked the rod glands and they are well back from intruding into the cylinder bore. I said originally I thought it was bottoming out by 3mm, but I think now it is more like 2mm, so I have made a spacer to rectify this issue. I have discovered that the final tightening up of the rod into the crosshead does have a bearing on the clearance with regards to the conrod fouling on the brass gland washers, so I have now reamed them out quite a bit to give more clearance, after all by the time the gland packing material is in place this will hold the rod more or less in the centre.
Just as a matter of interest with regards to the crossheads running up and down the slides , when one has done the necessary packing on each of the top and bottom slides, what do you think is an acceptable up and down lift (clearance) on the crossheads?
Thanks for your help again,
Charles

Hi

To clarify. The two piece block is the rear of the block has the cylinder bottoms and piston glands as a separate plate.
Original block has cast cylinder bottoms and cast cross slide bar supports.

The plate on the underside of the block at the steam port in from the boiler is not referenced as the two part block.

Hi Lynn,
Thanks for that, I did wonder that because Elwood referred to the the block with a 'separate rear cover plate'
Kind regards,
Charles

mea culpa!

As Lynn said the cylinder has a separate rear face. I am not sure if the old design also had a „bottom“ cover plate, at least the new design has it. But I was referring to the rear cover plate.

My block looks like that:



What you see is the rear side of the block looking forward, the cover with the gland fittings is bolted to that face. And the block looks much more rusty on the picture than it really was!

Before mounting I brought the block to a engine overhaul shop to have it pressure washed to get rid of possible dirt, casting sand and swarf inside the cylinder block.

Hope this helps...

Cheers

Elwood

Thanks Elwood,
I also found a a very large amount casting sand and swarf in my block, I collected most of it and it filled about 1/3 of a tea cup, and that was only what I was able to collect on the bench!
I spent most of a morning, fettling, cleaning and pressure washing my block ( After looking at the forum and somebodies detailed photos of the a rather nasty remains of a sharp bit of iron casting that was found in one of the cambers) I also removed this piece and cleaned the edge up as best as possible.
Because I have a lot of experience with rebuilding vintage engines I thought the cylinder bores were quite rough (turning marks) so I purchase two 'Silicon Carbide Flex-Hone' ( 3 3/4 & 2 1/2 dia. and 120 grit) which made a lovely job of the cylinder bores. These were quite expensive and I will not do not have a lot of use for them (especially the small one) so if anybody would like to borrow them, providing they pay the postage , they would be most welcome to borrow them.
Thanks for your help again
Charles

The reason it is threaded is so it can be set at the right length relative to the crankshaft. It is not meant to be screwed right in & the grub screws lock it in place so the piston doesn't rotate & alter the setting. I remember the instructions were silent on this so I measured it all up and on the early blocks the piston should be set so that the front edge of the piston lines up with the back edge of the front steam ports. With the throw on the crank it should line up with the back port in the same way. I know it sort of feels right to screw something right in but it was made this way to allow for any difference in length between the block & the crank. It won't breath right any other way & you won't get maximum power from the engine. How the engine breathes and timing so critical, fortunately timing is really easy to set up & adjust on these engines. Regarding breathing I put a small chamfer on the leading & trailing edge of the HP & LP valve blocks and it has made a big difference to the pulling power. With the new open blocks this may not be required. Just check that the valve ports can have a free flow of steam with each stroke of the block & that there is no overlap as this create turbulence which looses power. I have to say, once run in, they are fantastic engines to drive.

Hi Steve,

thanks for the answer but I assume that it reflects to the "old" cylinder block. The instruction on my kit states to "screw the crosshead tightly onto the Piston Rod" I assume that, as the steam ports of the two piece cylinder block are milled/drilled, the position is defined more exactly than with the old block where the ports were cast, so that any change or shift of the core during casting might change the position. Now, with a more closely guarded position of the port and only minimal deviation from ideal position (mounting bolt) this adjustment would not be necessary anymore.

But it might be interesting to check just the same. knowing the thickness of the piston and some more dimensions taking from the drawings one would be able to determine the final or top ded center position of the piston in the bore, which in turn can be measured easily on the engine. As we know the stroke of the crankshaft and the distance of inside edges of the front and rear steam ports are in the machining drawing of the block it should be easy also to determine the steam port overlap - if any - at the bottom dead center, too.

I am quite sure that on my engine wiith the piston rods screwed tight onto the cross heads the piston at TDC does not cover the steam port. I think I even have a picture of that, but cannot find it at this moment.

Or did I misunderstand your comments, Steve? If so, sorry.

Cheers

Elwood