Club 80-90 Forums

Ay up,
I've got low compression on all cylinders (just under 8bar) so I've bit the bullet and taken the heads off expecting to find something obviously wrong. However, apart from an oily black coking the valves look ok. However, looks aren't everything. Ideally I'd like to test these heads as I suspect they are the culprit as she would always burn oil (very apparent on start up).
The only potential fly in the ointment is that I can fit a 0.2mm feeler between the side of the piston and the barrel (which according to Haynes is the wear limit). I'm hoping I've mis-interpreted because these pistons and barrels are only 3 years old (GSF), surely they can't of worn already .
Sorry, I've ranted on, any advice greatly appreciated, and if anyone has done any leak tests on the heads (off the engine) then I'd like to learn more .
Cheerz

Oh dear... a compression test doesnt really work all that well in the wbx IMHO.. if they were all even and it isnt burning much oil, not pressurising the cooling system, then you should have left them on...

Clean the valves, lap them back in with a little fine grinding paste, MAKE sure you clean all gringing paste away REALLY well..
When refitting the rockers, set the adjusters to 1/2 (half) turn from just touching...

Oil smoke on cold start up wont be from the valve giudes, not generally in any case... its from the piston rings...

What make were the piston and barrels?
Are there wear ridges on the barrels? or nice and smooth?
Buy some gen rings to refit to the pistons.. BUT check what dimensions they are on the pistond first as there are some pistons out there with different thickness rings..
Make sure the gaps are positioned at 120 degrees from each other..

Thanks for the reply, I was beginning to think this is a bit too techy.
Well the heads are off now and I'm gonna get a local engineer http://www.daverushton.co.uk/default.html" onclick="window.open(this.href);return false; to give them the once over (he seems to think that the guides will be worn) and on closer inspection there is some lateral float between valve and guide.
Regarding the oil and the piston rings, I bought the pistons and barrels from GSF and I think they were Mahle. As I said earlier there is at least 0.2mm between the piston side and the barrel??? Could it be possible the piston or barrel has worn (GSF quality strikes again?)
There aren't any wear ridges on the barrels they are smooth and the original cross hatching from the manufacturing process is still present.
What do you mean by gen rings, (generic)? What type of rings works best when using LPG?
Cheerz

Gen = genuine......

Mahle are good quality..
Any idea what the gap was when they were installed originaly?


Bentley (official VW USA manual) states wear limits as...
Top ring 0.9mm
middle ring 0.9mm
Oil scraper 0.95mm

0.2 cant be right as a wear limit, that NEW...

Its poss when it was run in last time it was too gentle.. needs some load to break the rings in.. I would either deglaze barrels with a fine deglazing tool, or gentle rotational rub with oiled 800 grit wet and dry... your engine man can wizz over them in seconds...

Of course it does (gen).
I've got the ol' mans honing tool but I'm still baffled by this 0.2mm wear limit clearance between the side of the piston and barrel, not sure what the original gap was (doh) if I haven't got my wires crossed then it sounds like I've got worn barrels or pistons or both. I think I'll add this question to the list when I go to see the engineer.
Thanks for the data on the gap clearances
Cheerz

New ones wouldnt be much less than .2 so I'd ignore that...

Valve guides.. measured at the head with stem flush with top of guide.. 1.2mm max

Just a bit of an update. Had the heads off and inspected by Dave Rushton in Stoke-on-Trent. Valve guides were (surprisingly) ok, but the seats and valves were worn and coked up, these were re-cut and de-coked and looked like new when I picked them up.
I've learnt alot about pistons, rings, barrels in the last few days - many thanks for your help fellas - great forum!

Then engine is back in and running, once I've done a few miles I'll check the compression, but I can tell by the cranking speed it's improved.

By the way, if anyone is in the Staffs/Cheshire area and need any engineering work then Dave knows his stuff (either that or he's a good bullshi**er), he gets my thumbs up!

Cheerz

Where are you measuring clearance between piston and barrel? Around the crown will be very loose, and it's supposed to be. You need to measure down at the bottom edge of the skirt, slip the piston inside the barrel and you should be able to get a .001-.002" feeler gauge in at the edge of the skirt. 0.2mm (.008") would be very loose if you measured it there.

Thanks for that info, what you say is exactly what Dave Rushton said. Thanks for your response.