| Author | Message | ||
     Mark Taxis Experienced User Username: mark_taxis Post Number: 23 Registered: 4-2007 |
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 | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 576 Registered: 6-2006 |
Head and sump off I'm afraid  | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 578 Registered: 6-2006 |
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?  | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 24 Registered: 4-2007 |
thank you Paul , I will look at that before I go further, the thought of trying to remove the head is somewhat daunting! | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 2124 Registered: 4-2003 |
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. | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 585 Registered: 6-2006 |
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! | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 25 Registered: 4-2007 |
Many thanks to you all for your input, I will check valve clearances first - especially the exhaust and see what result that gives. | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 26 Registered: 4-2007 |
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. | ||
| Paul Yorke Grand Master Username: paul_yorke Post Number: 587 Registered: 6-2006 |
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 | ||
| Mark Taxis Experienced User Username: mark_taxis Post Number: 27 Registered: 4-2007 |
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 |
