Pre War Pistons

[AlanBartlett]

Hello all,
As I briefly explained in the Weasel update. The Saloon had developed a lack of compression during Goodwood use. It would pick up on two and run well, but give a poor idle. Once back home I went through the following options, giving it a compression test, revealed a low reading and further investigating of the valves revealed nothing jumping out at me. So I took the plunge to remove the barrel to see if anything was a miss.

This revealed a broken piston ring, on the top compression ring which saw lots of wear in the land and the top of the crown being splayed out where the broken piece of ring had moved side by side and widened the ring slot. So now that piston is pretty much scrap.
When I rebuilt the engine about ten years ago now as many of you will remember. I used the Std 8hp pistons in a set of 7hp barrels. So I am right on the limit for not being able to go to a size bigger. However I do have another set of std 8hp pistons. Now these are from a stationary engine I acquired with the job lot of lorry bits.

It would appear the pistons have the same part numbers. Though strangely the piston is machined from what I gather as was to have three compression rings instead of the two and an oil control ring. Why would that be? I would have thought an stationary engine would be doing the same job and would require an oil ring to control how much oil is going around the bores. My plan was to have the pistons machined to take a control ring but sadly on this one there is not enough meat at the bottom to achieve this. Would anyone have any thoughts?

Alan

[Forumadmin]

Not sure I can answer specifically about the 7 and 8 hp. Fitting piston rings does has some generic procedures to follow. You must check the gap of the rings at the top and bottom of the stroke and make sure they are within spec. That spec depends on bore and to a lesser extent on materials but 7 thou should be about right, anything less and it is probably too tight, more than 15 and it is likely to start to smoke.
Also check the wear in the slot after cleaning out any carbon with and old ring. viewtopic.php?p=46700&hilit=rings#p46700
download/file.php?id=26405&mode=view
download/file.php?id=26404&mode=view

Always use a good ring compressor and extractor. The rings can easily be broken on assembly. They can also have a top and bottom, so make sure they are the correct way around.

There are also some checks you can make on pistons such as the gap required for expansion. Sometimes bores can become oval and pistons are often oval anyway by design across the gudgeon pin.

[Tony Fearn]

Hi Alan, and Season's Greetings to you both as well as to all others who read this post.
Unfortunately I don't know anything about pistons as my engines were done by professionals some time ago.
Tony.

[James Baxter]

Trying to follow you here....
So you want to run 2 compression and 2 oil control, like the original 8hp instead of the 3 compression?
but not enough room to wider a compression groove into an oil control?
1st choice find a narrow oil control - modern ones are narrower than old Jowett.
2nd choice - fit it as it is. I dont think you will have a problem.....

Try to get a hone tool up the bore, to aid the new rings. New rings in an established bore is not recommended.
But new honing captures a bit of oil, and looks after the rings until bedded in.

[BarryCambs]

Hi Alan

Back in the day, some stationary engines were bored to give a greater piston to bore gap to try and stop problems with them starting at full power/revs from cold - fire pumps being a good example. I wonder if not having an oil control ring was something similar? I'm on 8hp pistons too, but I can't remember if the original 7hp pistons had a control ring? The office has been packed up due to building work, so I can't find my parts list to see what that says

[AlanBartlett]

Hello All,
I'll try to get this in order;
Keith
I'm happy with gapping and sorting the rings. Its about a couple thou per inch, ill glance back at my notes.

Tony
Seasons Greetings to you Tony & Hilary too.

James :
Basically I have a replacement piston I would like to fit in place of the damaged piston I've taken out. The original 8hp piston had 2 compression rings and 1 Oil Control ring.
The Replacement 8hp piston is from a stationary engine which has 3 Compression rings the 2 at the top and a third at the bottom, which I'm puzzled as to why it would have a third comp ring and no provision for an oil control ring, which from what Barry has explained makes sense. I think finding a thinner ring would be the ideal outcome as it has been running well with the original configuration of rings. Though I'm also wondering how much difference it would make to run three compression rings? My guess is the getting oil where it needs to be. Your right about the honing while its out a bit of freshen up will hopefully help.

Barry:
You explanation makes sense, from what I gather they were three ring pistons two compression and an oil ring. It would be a shame and expensive to have to sleeve and track down other pistons especially as the engine has done so well. I'm hoping this will be a good way forward. As I'm elbow deep in the weasel engine rebuild, having two on the go would make my wallet wince!