Not been round here much for a while as the M5 has been sat in the garage while I found time to look into the low compression on cylinder 2. As the head was going to have to come off and the car has done 151500 miles I figured I may as well pull the whole engine apart and check everything over, as these things don't fix themselves and I wanted to take it apart while it was still in one piece...
The reason for lack of compression was easily found - the exhaust valves on cylinder 2 were both seating poorly (note to self - must check that they aren't bent), but perhaps it's more interesting to relate what the rest of the engine was like after close on 20 years and over 150k miles. From the top down:
Inlet valves and guides all fine.
Exhaust valves fine but all guides worn to upper limit
Slight marks on one cam lobe
Slight wear on timing chain tensioner
Bores and pistons show no appreciable wear
Big end bearing shell upper halves just starting to wear through first layer of material - mains all perfect
Original clutch - just about worn out
So pending detailed measurements it looks like the bottom end just needs a hone, new rings and shells (mind you at current BM prices that's about £1000 of parts..) and I'll also throw in new chain, tensioners, rod bolts and so on, but basically the bottom end is still in decent shape
Top end is a bit more of a problem and I could do with some input here. Exhaust guides are shot really, but the valves are fine - standard factory procedure to deal with wear is to ream the guide and fit and oversize valve - but these are £120 each - that's £1,500 of valves - crazy - so I can see two other options - sleeve the existing guides, or replace them. A quick bit of internet research found VAC Motorsports do guides (there may be others) but has anybody used these and are they any good. I have an expert engine shop that can swap the guides if that's the way forward, but as the guides are more like £10 each it does look like the way to go - has anyone anyone done this?
Just note that the M10 engine uses the same guides as the S38. I used those in my b36 and they were 100%.
You know that when you ask for guides for an e30 318 the price is automatically 5x less than if you were to ask for an M5's guides
VAC has an excellent reputation for S14 and S38 engine work on this side of the pond. On a side note I have seen valves (standard sized) for as little as $31 Cdn (@ £20) each made by Intervalves INTERVALVES SA who are a swiss company. Not sure whether they make oversize stemmed versions. I've also found these. www.IPG-Supertech.com - Supertech Performance Valvetrain Not sure of the quality.
Replacing valve guides on the S38 head is not a trivial matter. It involves heating the whole head, drifting out the old guides, drifting in the new guides and properly decking/crowning the new guide with a special tool bit or an end mill. Not something you want to screw up. The logic is that the aluminum head expands more than the brass guide. One of the more effective methods I have come across involves boring them slightly smaller than their OD to relieve internal pressure to about 3/4 of their overall length, which leaves a step to drive them from there. Trying to drive them out whole will draw the guide boss and will leave large marks in the boss. Head should be heated and guides should be frozen also during install. Some guys even use liquid nitrogen on the guides and a bold that is inserted in the guide hole for the install. Guides should be replaced with ones that are slightly larger than the originals, (around .0005) The S14 and S38 guides have a tight (high interference) press fit which is the reason the factory recommends the oversize valve stem method.
... a little more progress. I spoke to Serdi today - guides are £5.85 each and to supply and fit all the exhaust valves is approx £270 plus VAT, so that looks like the way forward.
I can appreciate why the factory would prefer to supply oversize valves as it must be a simpler engineering solution to ream the guides and fit new valves, but Serdi are aware of the need to heat the head etc so I wouldn't have any worries having them do the job (though my other engine place does Aston stuff, and again to replace guides those, the head needs to be heated etc, so it wouldn't be a problem for them either). If new valves weren't such silly money I'd probably have gone down this route.
It doesn't appear that the valves are bent so I can only think that the guides were bad enough in no2 cylinder to let the valves flop around and chew the seats up a bit - I did notice that the seats in cylinder 2 was noticeably wider than all the others.
Finally, for now, a few pictures:
Engine out - easy enough to dismantle everything but a pig to wriggle out of the hole!
Head looks fine, just a bit of carbon - these are cylinders 1 and 2 - you might just be able to make out the fatter looking seats in number 2.
Bead balsted a few of the valves, inlets were pretty clean anyway
One of the pistons - original machining marks still clearly visible so hoping that these are all still in spec. there's a bit of carbon on these too, but no broken rings and they are still a good fit in the grooves
Big end bearings just starting to show some wear
Close up of number 4 - the worst one, but for over 150,000 miles not bad!
__________________ E34 M5 Touring, March '95, 100% original, Memory seats, Electric roof, 6-speed, 280.000 km, Fjord Grau. Renewed: Front suspension and disks, Springs and EDC's, Steeringbox and linkages, Crankshaft bearings, bigend bearings, Ignition coils and guides, clutch, rims (m-paralell) and tyres, All vacuum hoses, Waterpump, thermostat, black powerflex front and rear. Buildsheet / Touring registry