Low compression on one cylinder - next steps?

Well it went back a few weekends ago with the cleaned up head and a new gasket.

It's had the rocker adjustments done cold (+2/1000ths over the hot settings as mentioned elsewhere on here) and been turned over without exhaust/carb.

Sadly cylinder 3 remains very low. It was initally tested with a slightly dodgy compression tester, so this weekend it's been re-tested with a good tester and came up as follows:

Cold/Dry (psi)
1 - 130
2 - 150
3 - 25
4 - 120

Cold/Oil added to cylinders (psi)
1 - 210
2 - 210
3 - 130
4 - 180

Whilst they all went up I'm assuming the increase on cylinder 3 points to a bottom end issue?

I'm hoping to recheck with a leak-down tester on Wednesday to fully check it's the bottom end.

Given the van stood for a while before I got it and has only been run on the drive since I've got it and that the bores looked OK - Is the last "easy" thing to consider a stuck ring? Is it worth trying any of the "fill the cyclinder with XX" type suggestions?

Or is it time to call it a day and decide between fitting new rings with the engine in situ or thinking about a rebuild/ebay short engine?

Cheers All

Paul.
 
Bummer:(
The results do show a ring problem on No3 I'm afraid, but it's very low the result you would expect with a sticky valve. But if you're sure they are OK and the new gasket is doing it's job then there's only one way forward which is pop the pistons to find out. Have you started the engine since re-fitting the head? does it belch out blue smoke? The reason I'm asking is that when we swapped the head on Bernie and Heidi's van, Gladys, because of valve issues we got similar pressure readings to yours but once running it was perfect. Must have been either a sticky valve or my compression tester.
 
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Removing the piston:(, as Panky says, and cleaning out the grooves manually should fix stuck rings, however if a good soaking with XX or MMO works it could save the bother.
 
Thanks both!

Engine hasn't been run since the head went back on. We only got it back together enough to compression test, with the low compression still on the same cylinder as before we assumed the head clean-up hadn't fixed the issue.

Will try the leak-down tester today hopefully, which should hopefully confirm or rule out valves.
 
Quick update

The leak down test still pointed to bottom end on cylinder 3 - all air noise was in the dipstick, none in inlet/exhaust manifolds or radiator.

Have dropped MMO down all cylinders twice this week, tomorrow's plan (weather permitting) is to get it back to a running state. Run it, and recheck the rocker clearances with the engine hot, if it doesn't magically sort itself then it's time to strip it down on put a short engine in I guess.
 
Went to run it up to temperature 2 weekends ago and as ever it didn't go entirely smoothly.
Petrol absolutely flooded from the carb bowl overflow - so we spent a while fitting the service kit to the carb instead.
A lot of crap in the bowl and at least one blocked jet, so good to get done.

So last weekend it actually got started and run up to temperature. It started better and ran better probably due to the cab servicing.
Sadly though a hot compression test still returned 130, 150, 30, 130 psi.

Coupled with the leak down test I think it's going to have to get pulled apart.
 
Sadly I have to agree:( You've done everything you can to check everything but now there's only thing one left to do, with any luck (and you're due for some) it will just be new rings and a hone.
 
dodgeram : Initially a couple of ounces down each cylinder a couple of days apart as above. But I also gave it another shot this weekend just gone - I put about an inch of fluid in cylinder 3 and left it for about 24 hours at which point there was nothing remaining in the cylinder and running it up to temperature made no difference.

jimberlake/panky: Thanks. One to bear in mind and will chat with my more engine happy helper!

Trying to schedule a date with my friend to pull it apart
 
That's normal, still think it's the valves:rolleyes:;)
Hopefully it didn't damage the bores.
 
OK, I've measured the No 3 piston which is out using the info kindly supplied by Panky : "The measuring point is 15.8mm from the bottom of the piston and 90 deg to the gudgeon pin. A standard piston should measure between 81.514mm and 81.565mm (the range across all piston grades)"

Bearing in mind it's our first attempt at measuring something like this we got 81.54 mm at the Panky Point. So it would appear to be standard pistons.

Only slight concern is that it may not be 100% round. At the same height as above but in-line with the pin we got 81.33mm. Near the oil rings we got 81.51mm and 81.04.

The bores don't have any heavy scoring as shown and similarly the pistons. But the piston looks like some surfaces have been rubbing.

Given I'm looking for the fastest route to getting it running and MOTable rather than perfection - does any of this rule out just a minor hone and new standard rings?

bore1.jpg bore2.jpg piston.jpg
 
If I was in the same situation that is what I would do, not ideal but the engine should last many miles with a hone and new rings. The thing that would concern me more is if the ring grooves were worn allowing the rings to slop around - but from that pic the ones we can see look nice and sharp.
To hone out the bores get some medium emery cloth (not wet and dry) and, if your hand is small enough, give the bores a good scrubbing with that. Alternatively make up a dummy piston from layers of plywood and wrap the emery around it to do the job.
 
Ideally yes but for just one engine a sheet of emery is a lot cheaper, and I'm a tight old git;)
 
A light hone won't get rid of the wear ridge at the top of the bore. New rings will smash themselves against the ridge unless you can get "ridge dodger" stepped top rings to avoid this.
If you can catch the wear ridge with a finger nail it's too big. Further down the bore looks a bit scabby too.
 
Just so you know, pistons are meant to be oval and tapered, thermal expansion is what makes them round.
That bore looks borderline for a rebore, but see how it looks/measures after a hone.
 
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