m103 compression interpretations + valve guide questions
 

Well my Brabus 3.6 has eaten its way through yet another headgasket :(, thankfully I have an MLS gasket that PUMPISH made for me. I've been losing oil in this car at the same rate (~1L/300mi) I had done before the rebuild last summer (installed gapless 2nd rings having re-bored a new block out to 92mm). I gave it a compression test today with the following results for cylinder numbers
1: 12.5 bar
2: 12 bar
3: 11.5bar
4: 8.5 bar
5: 13 bar
6: 10.75 bar.
I expected adjacent cylinders to be low on compression. I added a teaspoon of oil to cylinders 4 and 6 and re-checked the values but the compression changed by maybe a quarter of a bar which I don't deem to be significant considering the gauge I used is a Gunson's gauge. My car has manual valve lash adjustment which I adjusted less than 3,000 miles ago. The valve guides were changed about 20,000 miles ago; the last time I had the head off (~5,000 mi) my engineer told me that the guides were fine.

The car idles slowly (just over 500rpm) and rather grumpily with a slight misfire that clears up at around 2000 RPM (it no longer has the hesitation which I have previously posted about though). On the motorway there is little perceivable difference between how it runs now and how it ran 30,000 mi ago. It's very torquey and 90 to 140 is still very impressive. Low down, from a standstill I guess it is slightly slower than its optimum.

I'm wondering whether the valve guides on cylinder 4 are shot (certainly I remember them being darker in colour than the others) and whether this is the cause of my oil consumption. Furthermore, if this is the case then perhaps the worn guides are preventing the valves seating properly? The spark plugs are all dry, the 4th plug has a burgundy colour and the 5th is black, sooty. I'll be opening the engine up tomorrow and having my engineer check the bores for wear and piston deck height (for a possible bent conrod) on Monday. Ideas & guidance would be much appreciated.

A picture speaks a thousand words. Anyone care to bite?

Headgasket break
http://www.peachparts.com/shopforum/...&pictureid=326
Similar breaks are repeated in a couple of other places, I'm guessing this is where my coolant is disappearing into the combustion chamber.

Underside of the head
http://www.peachparts.com/shopforum/...&pictureid=328
I've already noticed some dirt on the under side of number 5's exhaust valve, the part that is cut and should seal with the seat, I doubt this can seal properly in its current condition. My fuel distributor was overhauled by Ken Mills Injection Limited a year or so ago. Perhaps cylinder 5 is this this sooty due to a leaking injector?

A typical cylinder
http://www.peachparts.com/shopforum/...&pictureid=323
The visible strips look to be what Mercedes describe as friction marks due to frequent short journeys and is not a worry according to their literature.

As for the valve stem seals, I replaced them when I put the engine back together last summer. I've just checked its disc diameter, they are standard Mercedes valves. I will check the valve dimensions against the Merc specs. Anything else to check for on those? How about the seats, when do they need changing?

A small break like that in a headgasket should cause such a coolant leak as you described

When you rebuilt the motor did you torque the bolts in the right sequence?

As for the specs measure what you got and put it agianst mercedes allows.

#5 looks like oil consumption to me. Most likely an intake valve guide. I'd check the freeplay with the valve lifted and only worry about the bottom end if the guide is tight. Can you get a close pic of the #5 piston? What cylinders were getting coolant into them?

Oracle I did tighten them in sequence, although I have converted to head studs so I tightened in stages 45 ft/lbs, 60 ft/lbs, then 85 ft/lbs.

Here is piston 5
http://www.peachparts.com/shopforum/...&pictureid=335

& combustion chamber 5
http://www.peachparts.com/shopforum/...&pictureid=339

Inlet port 2 - same as 1 & 3
http://www.peachparts.com/shopforum/...&pictureid=336

Inlet port 5
http://www.peachparts.com/shopforum/...&pictureid=337

Inlet ports 4 & 6 are dirtier than 2 but better than 5. I will be measuring the valves today. If any are out of spec should I order both a valve and valve guide in repair size? How about changing the valve seats?

The guide is a lot softer material than the valve so I would be very surprised if the valve showed any measurabe amount of wear. I would feel comfortable ordering standard size guides...all of them. Don't just pick and choose, replace them all. I would say #5 is definately burning a lot of oil. It is very probable that the oil in the cylinder caused detonation, which in turn blew out the head gasket. The 103 just won't take much detonation before blowing the head gasket. The steam slots between the cylinders give the pressure an out and when detonating it takes the out. With the 3.6 having such a small area in the fire ring it is even more prone to blow the gasket. The upside being that a head gasket is cheaper than buying pistons and doing a bottom end rebuild.

The valve seats should be fine unless the head has had a bunch of rebuilds. After the new guides are installed you just need to cut the seats enough to get new surfaces for the valves to seat on. A nice multi-angle grind on the seats may pick up a bit of low lift flow while you're at it but not a necessity by any means.

When torque-ing the head I like to do an initial pass about 1/2 of the first stage. Starting with 20ft/lbs gets the head snug on the block without as much flex as cranking the center bots to 45lb/ft right off the bat. I don't think head bolt torque caused the failure, I just don't like to put 45ft/lbs on the first few bolts with such a long aluminum head.

An engineers verdict
 

I thought I was onto something yesterday as I was dissembling the head. The intake valve guides were all tight but the exhaust guides had some play in them. However my engineer inspected them and told me that they're all perfect and the extra play allows for the expansion of the much hotter exhaust valves. The valves themselves all measured up within tolerances. I must reiterate that he is used by Mercedes of Colindale and so has seen many m103 heads.

He then inspected the block, immediately telling me that there is no wear in it but that it has become glazed and looks like a 100,000 mi engine (despite having a new bore 5,000 mi ago). He told me to lap the valves in again and get the pistons to him. The bore will be honed and a new set of rings put on. The engine was running very rich for 500 mi after the rebuild which he thinks caused the glazing.

P.S. the new m103 valve stem seals are all the same (both intake and exhaust), just bought them from Mercedes. An improvement or cost-cutting?!

how rich a tune are we talking here?????? for the issue tobecome a worry?
i ask cause i have twinturbo kit on my m103 and the tune is far from spot on
a tad rich on idle, just a bit. then midrange on full throt its abit rich, but i assume its fine further on.
so am i to worry abt it.?

I just finished a valve job on my M103 300SE. The head bolts are to be tightened to 50 something ft-lbs and then angle tightened 90 deg. twice. When I did the final 90 deg., it felt much more than 85 ft-lbs. I'd like to suggest you remove the studs and use new bolts.

To replace the guides, use a cannister of liquid CO2. Shoot the CO2 down the guides and they will drop right out, with a little help. Cool off the new guides and they will drop right in. Use proper heavy gloves when handling CO2 because you will get extreme frostbite.

I was able to use this technique on one or two new guides because I had damaged one and another was not in all the way.

Jay - as long as its within Merc specs (there's a sticker under your bonnet showing this) it should be fine, my mixture was over 10% CO, that's when you have to worry.

Warren I think the simply headgasket failed, it wasn't due to the head not being correctly held on. May guides are perfect according to the same engineer that the dealers use, as they should be having been previously replaced.

My engineer has informed me that my pistons are all 0.002" too small with piston number #5 0.004" too small. I now need to source some pistons for the vehicle. Seems strange that the pistons have worn and the bore hasn't though.

I'd suggest buying the technica data book for your car but Brabus has modified your engine, either by boring and/or stroking. Please contact them for the proper piston specs.

I've overhauled my M180 220S engine with new pistons and the cyls are bored to the 1st oversize spec but the pistons are all slightly different sizes.

I still think you need to revert to bolts.