Pistons

[Marinus]

Hi all,

I am now at the part of making the pistons for TICH's cylinders and wanted to know how I should machine it to get it to fit. I bought some LG2 gunmetal rods yesterday and I don't know how to make a piston the right fit.
The pistons will have graphite yarn around them , so how should I mae them to fit?

Any help would be appriciated

[tel]

Pistons are best roughed out something over sized, then mounted on there piston rods (held in a collet for preference) for final turning. Just keep taking VERY fine cuts and use the cylinder as a gauge. You do have a little leeway on the fit since you are using graphite packing - a couple of thou undersize won't matter much.

[steamer]

Wot Tel said!

Dave

[b.lindsey]

Marinus, I am assuming you have the cylinders bored/honed/lapped to their finished size...else your efforts may be in vain.

Bill

[Marinus]

Thank you all for the reply. That helped alot, thanks Tel
Bill.. yes it is honed to final size

[Stuart]

for a loco make the 0.001 inch smaller that the bore when using soft packing  a couple of thou for CI rings in CI cylinders

if you make them to good it will bind up when it get hot , its the packing coupled with the steam oil that give you the seal

to coin a phrase for our lady doctor Steam engine need to be knackered to run not really but they do need a bit of clearance

Stuart

[Marinus]

Thank you for teh reply Stuart. Now I am convident that I can make a piston

[Steam Haulage]

"Steam engine need to be knackered to run" !!

Don't let anyone from Swindon hear you say that. It's a good job Wiltshire is a fair distance from Derbyshire.

JerryNotts

[derekwarner]

Best qualify this by bore sizing........0.001"/inch is realistic for model engines from say 0.5" to 3" bore .......from there upwards the clearance should diminish  ...Derek

[Stuart]

Derek

A the OP said he was building a tich loco so as not to complicate the issue I quoted the fit to suit his build

Stuart

[tel]

13/16" bore from memory. 

[derekwarner]

Stuart....with my age ....I was trained  with Christian units of measurement  ...being " & parts there of  .......

It was not my intention to quote song & verse from BS.1916 Part 1 "Limits of Fits"........just to offer a humble clearance concept of 0.001"/inch is realistic for model engines

Further to this I do not believe BS.1916 Part 1 "Limits of Fits"........should be referenced by younger [inexperienced] engineers as the Standard does not consider any  temperature gradient .....Derek

[Marinus]

Thank you all for the reply.
11/16" bore Tel
I will make the pistons 0.02mm/0.001" smaller to fit.

[tel]

The best test of piston fit is to either block one end with your thumb, hold vertically and with the piston/rod inserted it should not sink down into the bore until you remove said thumb. Then a little 'suck and blow' test - DO NOT INSERT THE PISTON ROD END IN YOUR MOUTH for this one! A gentle sucking and blowing should see the piston moving back and forth.

[Marinus]

Tell, is that with the packing or without?