Which model Thwaites is this?

[cobbadog]

Yes, very glad you are heading in the right direction for sure. Start soaking your piston and gudgeon in automatic transmission oil to help free it up. Do a Google search for a company that either makes or supplies rings and you will be able to buy what you need. Here in Oz there are a couple of companies that sell stuff like that. Even a couple of adventurous machinists make their own piston rings from old cast iron pipes. But there is a formula for doing this with making them a bit oversized and then heat treatment and then cutting the opening. Scary stuff to me but you will be surprised maybe even an engine reconditioner will have a ring to suit.
I would not put it back together without a ring on the bottom. It keeps the skirt of the piston off the bore. Refresh my failing memory please, what engine is in your Thwaites? If you have a set of digital calipers you can accurately measure the inside diameter of the ring gland to give you the ID of the ring, then the ring gland width and finally the bore size and a ring will be found to suit. Maybe it will need to be an oil ring but if not a compression ring will be fine.

[cobbadog]

I would fit a ring to ALL ring glands. This will ensure that you get no marks from the skirt of the piston. It is there for a reason so I would find out if it should be an oil or compression ring in that position. I would guess an oil ring.

[SiH]

Thanks Cobba,
Will do as you set and get a hole new set of rings for each piston, and have seen them for 25 quid a piston,so pretty reasonable price eh . Its a Lister ST2 by the looks of it, no ID plate is a shame, but I have some calipers so will make sure of size before I order anything. Its all soaking at the mo, so should help free things off more too as you say. I’ll try and do a bit more this week, so will get back to you soon.
Cheers all, and have a good Sunday or ‘Bonne dimanche ‘ as they say here!

[cobbadog]

MY Liner Roughrider has a SR2 Lister and fortunately runs like a clock so I went the whole nine yards and a full service with all new filters, fuel, oil and air, new oil and I drained the fuel tank and put fresh fuel in before starting. I don't think that diesel goes off like petrol can but why risk it?
That is a good idea to use a micrometer to get the exact sizes of your bore and piston and the depth of the ring glands.
Was there any marks on the bottom of the skirt of the piston? This might have been what had been stuck. It is good to see that you are not rushing into it and allowing it to soak for longer will do no harm and will only help further down the track.

My current project is hitting a milestone next Sunday, the dustless sandblasting is going to be done, weather permitting. This is on our 1954 David Brown 30c Cropmaster tractor. I want it ready for our Rally in September.

[SiH]

Been a while,apologies boys, , and have progressed a little more with the engine. I’ve freed off both gudgeons pins and cleaned them all up, and also discovered one of the valve guides was broken due to a bent valve, but cant think if its inlet or outlet without looking . So I’ll buy new.
After measuring the cylinder bores, they are both just under 1mm wider in diameter than when new. What size rings should I order? Originals or oversize by 0.010 or upto 0.040 inch? Looking forward to getting it all back together and in use this year hopefully.

[cobbadog]

Slow but good progress. As for ring sizes, standard or 0.010" oversize I would measure the bore and the piston and write down the sizes and ask the supplier of the rings as to what is correct. I suspect that standard might be the go but dont take my word on that. Is there any markings stamped on top of the piston, sometimes years ago they used to do that.
I sometimes buy oversized rings for my mowers as they have worn bores but as they are only for Rallies and not use I then put them in place in the bore and measure the gap to ensure they are not too tight. This is something that you too should do before fitting the rings to the piston. Ring gap is based on the diameter of the bore and to work out the gap I Googled ring gap and found the answer that way.

[SiH]

Thanks Cobba, you are the second person that said they thought standard size, and as the wear isnt that bad I think I’ll go with that. Thanks for the ring gap info too. Right, better get on and order the bits then . Cheers for now.

[FOWLER MAN]

Hi,
If the piston is oversize it should, as has been said, be marked with the + size stamped on the crown.
You can only fit oversize rings if the pots have been bored and oversize pistons fitted.
You say the bore is just under 1 mm. bigger than standard, that may point to a plus size piston being fitted as +. 020" is + .51 mm. and
+ .030" is + .77 mm.
Allowing for running clearance your bores are possibly +.020
Don't forget to gap the rings near the bottom of the bore. If I remember rightly the rule of thumb for gaping the rings was .0045" for every inch of the bore diameter. That's slightly more than for water cooled engines as they run cooler.

[SiH]

Many thanks Fowler man (and Cobbadog),I didnt realise oversize piston rings meant oversize pistons. You guys have fantastic knowledge of it all, damn impressive, and apologies if I sounds clueless!
I cant see anything obvious on the pistons, but the better one has ‘BLPN090105N060T 4E JF’, then ‘CAMSHAFT SIDE 350041’ under that.
Thanks again for your time.

[SiH]

Could the ‘060T’ part mean 0.060”?