Piston Clearance - Air Pump

[Dhutch]

Daniel, can we see some pictures to get a better idea?
Assuming the liner is gunmetal and truly cylindrical then make the bucket of a similar material, otherwise it may need a re-bore or at least honing. I would have a look at leedsbronze.co.uk they do supply small pieces pro forma. Probably LB2 would be the best bet.

Regards
Jack

Ok. As said, I have not measured anything yet and your right in the need to confirm the state of the bore. Assuming it was true to start with, would a telescopic t-bar gauge be accurate enough to assess the current state of the liner?

I have just uploaded the photos to an album online; https://www.facebook.com/media/set/?set ... ad72a2e6ea


Daniel

[Lopez Mike]

If the clearance is more than .001", you might have an easier job of measuring the clearance by using a feeler gauge. You can do good work with a telescoping (snap) gauge but there are multiple steps in which errors can creep in.

[malcolmd]

Just a thought on the rings: 2stroke petrol engine is use cast iron rings with much larger ports (up to about15% of circumference) with no problems in much more extreme environments..,.so maybe you could too...(.and most piston valves are the same)
Mal

[Mike Rometer]

Just a thought on the rings: 2stroke petrol engine is use cast iron rings with much larger ports (up to about15% of circumference) with no problems in much more extreme environments..,.so maybe you could too...(.and most piston valves are the same)
Mal

As long as there is a position of the bore where you can fit a peg to the piston to stop the ring rotating.

[steamboatjack]

Folks,
I would not use C.I. Rings in a bronze liner. Weir's used rings made from “ebonite” a kind of hard rubber/plastic material which you had to put in boiling water to fit. I would suggest one of the modern plastics such as Teflon, Peek or Nylacast oilon would do if you had to use rings.
Regards
Jack

[Lopez Mike]

I have had good service out of plastic rings of various compositions as long as the bore was quite smooth. When I assemble a fresh C.I. engine with conventional rings there is a mutual wearing away of both surfaces that produces a mirror surface in a fairly short time. The dimensional changes are almost unmeasurable but the look and feel of the changes are obvious.

I would use Teflon or Acetal with no qualms as long as the either A: The bore was smooth from previous use, or B: you could lap the bore to a mirror like finish with a felt pads over the stones of a hand hone.

I once tried using teflon buttons on a C.I. engine to replace the snap rings at each end of the piston pin (gudgeon pin). In an at least slightly used engine it worked well but on a fresh bore, even one honed, the buttons wore away to fuzz rapidly with catastrophic results.

A consideration, however, would be any flexing of plastic rings when passing over ports. Metal rings are quite stiff and as long as you bevel the top and bottom edges of the ports they will pass over without incident. Plastics maybe not.

If I were doing rings I would heed Jack's admonition about compatibility and make some bronze rings with location pins to prevent a ring end lining up with a port and catching. All a lot of trouble.

My first choice would be the simple water grooves that have served well for ages. Most of the old and tried methods have worked well for good reasons. Bronze on bronze or iron on iron will outlast your interest in the matter.

[Dhutch]

As long as there is a position of the bore where you can fit a peg to the piston to stop the ring rotating.

That the issue, there are all round. Each port is only around 15 diameter, but the land between them is tiny and no enough to have a ring end pass through.

[Lopez Mike]

Sounds like simple grooves and a close fit will work for you. Again, this is a solution that came over on the ark I think.

[Dhutch]

As a bit of an update, in the end I sent the air pump away and had someone re-machine it.

What was done.
- Pump rebored, and bore honed.
- New piston and piston rod made, and lapped into bore.
- New valves made for both ends, including new valve guide pin at the base of the pump. (uppder valve is guides on piston rod)
- Piston shaft reduced circa 1mm to piston back into center of pump and remove fouling at bottom end of stroke.

This appears to have fixed the pump, and after a season while a strip and inspect is due, appear to be running fine.

Vacuum is still not what it should be however. Some testing using a large electric bilge pump suggests increasing the coolant flow (approximately doubling it) gives a significant improvement.

I think its always been a bit marginal, and on the very thin end of the wedge, the circ pump is also fairly worn. The question is how to increase it flow. It will be passing an amount, which remachining would recover, and it would also be feasible to over-bore it a noticeable amount, if not to twice it capacity. However it also currently draws the water in to the bottom on one stroke, and ports it round to the top on the second. The bore/pistonrod diameters are such that this means around half the capacity is ejected on in each stroke (the headside minus the rodside on one stoke, and the rode side on the other). However reconfiguring the valves it could be made to eject 50% more (headside one stroke, rod side the other) which is what the similar pump/engine setup in NB Tixal does. With a rebore we could I estimate get close to doubling the flow, pipework is also 15mm which feels a bit tight, so that would be upped to 22m or even 38mm.

I will post again when I have the various temperature, rpm, and flow rate recordings tabulated for consideration.

Daniel