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Mark Taxis
Experienced User
Username: mark_taxis

Post Number: 23
Registered: 4-2007
Posted on Friday, 14 May, 2010 - 08:32 am:   Edit PostDelete PostView Post/Check IP

One cylinder on my MK6 has very low compression, I suspect broken rings. I have poured a small amount of oil in the spark plug hole and retested which gave me good compression, so this lead me to think there is a problem with the rings.
In order to change the rings do I have to remove the head or can the piston be remved from the bottom of the block?
Mark
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Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 576
Registered: 6-2006
Posted on Friday, 14 May, 2010 - 08:34 am:   Edit PostDelete PostView Post/Check IP

Head and sump off I'm afraid :-(
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Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 578
Registered: 6-2006
Posted on Friday, 14 May, 2010 - 06:13 pm:   Edit PostDelete PostView Post/Check IP

Just a thought, could the oil run onto the exhaust valve and help seal that? Might be a tappet that's too tight?

Clutching at straws on your behalf? :-)
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Mark Taxis
Experienced User
Username: mark_taxis

Post Number: 24
Registered: 4-2007
Posted on Friday, 14 May, 2010 - 09:51 pm:   Edit PostDelete PostView Post/Check IP

thank you Paul , I will look at that before I go further, the thought of trying to remove the head is somewhat daunting!
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Richard Treacy
Grand Master
Username: richard_treacy

Post Number: 2124
Registered: 4-2003
Posted on Saturday, 15 May, 2010 - 05:38 pm:   Edit PostDelete PostView Post/Check IP

A note along Paul's lines. Those Postwar 6 exhaust valves do go tight and lift. They are usually left unadjusted for decades as they are fairly stable, but in some cases they need attention. If caught in time, a simple exhaust tappet adjustment can solve it before burning the valves out, although for a first-timer it seems a nightmare job even with mirrors and Houdini as your mentor. Next time is quite straightforward done blindfold.

The oil test is a bit hit and miss. Remember, if the compression is a ring-piston issue, then the piston is usually so shot that a rebuild is necessary. Low compression due to ring failure is almost unheard of on these engines, especialls since almost all MkVI and R-Typer motors have most compression rings broken soon after new or rebuild anyhow unless upgraded to full length liners. Only on the S1 did Crewe fit full-length liners from new, and Hey Presto.

I have enjoyed and suffered these motors for so long now, and the only seriously worrying causes of poor compression I have seen are, in order of frequency, 1. Blown head gasket 50% (fit an S1 UE5053 gasket to a 4 1/2 Mk VI or R-Type as a great improvement. 2. Cracked head, 25%. 3. Burned valves. 20%. 4. Piston/ring 5%, because poor compression barely even shows up with completely ruined pistons and all rings broken.

RT.
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Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 585
Registered: 6-2006
Posted on Saturday, 15 May, 2010 - 05:55 pm:   Edit PostDelete PostView Post/Check IP

I guess we can assume the the inlet valves are correctly adjusted to.

On the cuff liner front. There still seems to be quite a few running on the original set up. I've had 3 in the last 18 months. Shockingly, only one had damaged rings and that was only on two pistons!
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Mark Taxis
Experienced User
Username: mark_taxis

Post Number: 25
Registered: 4-2007
Posted on Monday, 17 May, 2010 - 10:44 am:   Edit PostDelete PostView Post/Check IP

Many thanks to you all for your input, I will check valve clearances first - especially the exhaust and see what result that gives.
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Mark Taxis
Experienced User
Username: mark_taxis

Post Number: 26
Registered: 4-2007
Posted on Wednesday, 19 May, 2010 - 08:07 am:   Edit PostDelete PostView Post/Check IP

I checked the valve clearance - all were out of adjustment so the engine is running a lot better now. Adjusting these valves is a lot easier on my special as I just have to undo 2 bolts to remove the side cover on the engine compartment, but still requires the dexterity of Houdini!!still got low compression on # 2 so I have to assume that the head will have to come off at some stage to investigate further.
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Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 587
Registered: 6-2006
Posted on Wednesday, 19 May, 2010 - 09:01 am:   Edit PostDelete PostView Post/Check IP

Mark were the clearances too small/ non existent ?

If far too tight then it may have been blowing exhaust gasses past the valve seat. Best ways, the seat is dirty and may reseal after it's been spinning for a while - worse ways, burnt seat and valve :-(
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Mark Taxis
Experienced User
Username: mark_taxis

Post Number: 27
Registered: 4-2007
Posted on Wednesday, 19 May, 2010 - 05:28 pm:   Edit PostDelete PostView Post/Check IP

Hi Paul
There was no clearance at all! Will run the car for a while then re-check the compression, - here is hoping that the valves and seats are not damaged

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