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loss of compression on number 1... Austin light 12 1934
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mid



Joined: 10 Jun 2008
Posts: 136
Location: Northampton

PostPosted: Sat Jan 02, 2010 9:44 pm    Post subject: loss of compression on number 1... Austin light 12 1934 Reply with quote

right... the symptoms:

noticed loss of power under load and noticed a fair bit of smoke. turns out the smoke is more like oil vapour coming from the breather hole at the bottom of the bell housing. car seems to start and run ok though. I've also lost the oil filler cap.

done a compression check and 2,3 and 4 are all about 90psi, number 1 however is about 30. this increases slightly when squirting a bit of oil in through the spark plug hole.

thoughts were a broken ring as this would explain the pressurising the crank case and causing the vapour and loss of the oil cap?? any other thoughts?

I've now removed the sump and the head, the sump was very gunged up but there was no signs of water. The valves look ok and i've put some oil on the pistons and its seemed to have stayed there. the bores also look good.

I assume the next step is to pull the number 1 piston to check on the rings. anyone else have any other ideas?

thanks in advance
martin.
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peter scott



Joined: 18 Dec 2007
Posts: 6229
Location: Edinburgh

PostPosted: Sat Jan 02, 2010 9:55 pm    Post subject: Re: loss of compression on number 1... Austin light 12 1934 Reply with quote

mid wrote:

thoughts were a broken ring as this would explain the pressurising the crank case and causing the vapour and loss of the oil cap?? any other thoughts?



Hi Martin,

Maybe more than just one ring on that piston.

Peter Sad
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1939 SS Jaguar 2 litre saloon
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Ray the rocker



Joined: 01 Aug 2008
Posts: 187
Location: south wales

PostPosted: Sat Jan 02, 2010 10:05 pm    Post subject: post subject Reply with quote

You don`t say if the engine is burning any oil? Backpressure has certainly put paid to your oil cap as the pressure has built up .i would suggest the oil control ring is still intact but the compression rings are more or less broken .Pull the piston as you suggest--just hope you can reclaim the piston before any damage.Good luck!!

Cheers--Ray the Rocker....
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mid



Joined: 10 Jun 2008
Posts: 136
Location: Northampton

PostPosted: Sat Jan 02, 2010 10:43 pm    Post subject: Reply with quote

engine is not burning any oil - if fact it still runs well but only struggles under load.

I'll try and get the piston out tomorrow and see where we go from there!
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mid



Joined: 10 Jun 2008
Posts: 136
Location: Northampton

PostPosted: Sun Jan 03, 2010 5:56 pm    Post subject: Reply with quote

seems as though this may be the issue then:



anyone know where i can get some new ones!

I'm amazed that they all stayed in place and have not damaged the bores or piston!
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Ashley



Joined: 02 Jan 2008
Posts: 1431
Location: Near Stroud, Glos

PostPosted: Sun Jan 03, 2010 6:15 pm    Post subject: Reply with quote

I'm sure Austin specialists will be able to help as will Thorntons of Shrewsbury if you give them relevant dimensions.

I usually take my pistons to an engine boring and grinding company to look the ring catalogue and find appropriate ones or, if there aren't any, to modify the pistons to take something more modern. While you're at it I'd do the lot, so it doesn't happen again sometime again soon.

Ash
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Ray the rocker



Joined: 01 Aug 2008
Posts: 187
Location: south wales

PostPosted: Sun Jan 03, 2010 9:12 pm    Post subject: post subject Reply with quote

great to know the problem is just the compression rings--but if you find rings to suit,make sure the piston is`nt stepped around the circumference of the ring groove as if the rings are fitted,they will act as a pump--picking up oil from the bore and finding it`s way to the top of the piston,thus creating oil burning combustion and blue smoke!!
Happy rebuilding!

Cheers---Ray the Rocker...
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mid



Joined: 10 Jun 2008
Posts: 136
Location: Northampton

PostPosted: Mon Feb 08, 2010 12:22 am    Post subject: Reply with quote

well i've eventually got the new rings in and the whole thing back together, new gaskets, fresh oil and fresh coolant - went to start the old girl up Saturday, did my usual of turning her over on the starter with no choke to get the petrol pump going then trying a second time with the choke out. choke seemed very stiff but it came out eventually so i put it down to a sticky cable, car eventually started, reved, then died. started again, reved and then died. noticed a hissing noise when she did start so went to investigate...

the choke had stuck on the throttle linkage... pulling the choke knob harder had allowed it to come free but had also cracked the carburettor casting in half!

the crack runs at about a 30 degree angle across the throttle butterfly!

It never rains and all that!

anyone know of any stockests of Zenith carb bits?
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