104.980 head work

[duxthe1]

So I bought another coupe recently. Got it for a song on Ebay. Well there is no such thing as a cheap Mercedes and this one had a problem with the #4 cylinder leaking combustion into the cooling system.

So off with the head. Sure enough #4 has a slight line of corrosion through the fire ring that almost looks like a scratch. Minute enough the motor actually ran ok, but still enough that pressure would build in the cooling system. There is also coolant residue half way up the intake manifold from it. Pretty yucky.

With the head off I found some interesting things. This isn't actually the original head. The car was built 4/90 but this head is the post 11/90 head. Smaller diameter valve stems and the cam thrust bearing is the middle cam bearing instead of half moons at the front.

Well I'm never one to leave well enough alone. Looking at the head the ports are pretty darn nice as MB cast them. The only place I see room for improvement is in the chamber. About 15-20% of the intake valve is significantly shrouded at low lift. A die grinder may have done the job of adding some clearance but for the sake of consistency I made a tool that would cut the additional clearance around the valve seat.

I used an old head gasket to determine how far out I could go and locked the cutter at that distance. I started out by using a cheapo drill to spin the cutter but the interrupted nature of the cut didn't give a very good result. I switched to using a T-handle by hand and got a much better cut. After cutting to depth by hand I switched back to the drill to get a better finish on the side of the cut.

First picture is the damage to the #4 cylinder. Next is the tool in a port. Last is the clearance gained by the operation.

[duxthe1]

More work on the head. This time gasket matching the exhaust ports. The manifolds match the gasket to the T but the ports in the head are noticeably smaller. In the pic you can see the carbon ring around an untouched port. This ~4mm ring represents how much smaller than the manifold that the port is.

This is just a lot of tedious time spend with a carbide cutter. I opened up the ports to match the gasket and then blended back towards to valves. Using a small spring loaded bore gauge allowed me to keep the ports a consistent size. I didn't touch the bowls or the SSR.

Next step is dropping the head off at my machinist for a skim cut and a valve job.

[duxthe1]

Dropped the head off today. Looks like a 3 week wait due to my machinist taking next week off. I don't trust anybody else to do the work so the wait is what it is.

Meanwhile I've been researching the cams. I have a 57B exhaust cam, pretty standard part. My math says 197 degrees duration @ 2mm lift. My intake cam, a 67B doesn't exist as far as I can tell. I've found over a dozen cam codes, but no 67 mentioned anywhere. It appears to have a 32 degree advancer typical of most 104s. If its like any other 104 cam I've seen numbers for, it would have 180 degrees duration @ 2mm lift.

I've heard talk that an exhaust cam could be swapped for an intake. This gets the gears turning in my head. If I could swap an exhaust cam in for an intake that would pick up ~17 degrees of duration. I was also thinking that with the extra duration I'd use a M111 or M119 advancer on it for only 20-22 degrees of advance / retard. This would keep the valve events in a reasonable range despite the extra duration. The duration increase would lessen dynamic compression but the head is going to be milled to clean up the damage to the #4 fire ring so it will pick up some compression there. I gave my machinist the minimum spec, but I'm not sure it will clean up at the min so I told him go as far as necessary and I'd deal with the clearance and valve timing issues.

Would love to hear some thoughts on this idea from the 104 gallery.

[duxthe1]

Ok here's what I figured out today....

First, the exhaust cam has a 4mm smaller base circle. That leaves 2mm that needs to be made up and I'm thinking that can be handled with shims.

Second, the cam advancer is fed oil through the camshaft flange through two holes drilled at an angle into the hollow center. I'll be able to make a jig to drill these holes properly into an exhaust cam.

Third, there are two holes in the flange that serve as drains for the advancer and they are the same size and position in the exhaust cam so nothing to do there. There is also a locating pin which is also in the correct position. The advancer I'm planning to use is off an M111 engine with (I think) 20 degrees advance. It's plunger is a couple mm longer. A few minutes in the lathe will take care of that. The tab for the cam sensor will have to be moved but should be a simple enough operation.

So far this exhaust to intake cam swap looks doable. Timing it all together may be a little tricky but I feel like it will be doable by setting the timing by the lift specs. My 104.980 has an idler gear below the intake cam. This may afford me an opportunity to correct the cam timing due to the head being milled. If I make a new spindle for this idler with an offset mounting hole I can rotate it to advance or retard the basic cam timing a few degrees.

I'm getting excited to get this motor back together!!!! I'll try to get some pictures soon.

[duxthe1]

Ok here's some pics.

First is an intake cam. For the sake of accuracy this is actually the M111 cam that donated the 20 degree advancer. You can see the two holes near the center that feed oil to the advancer.

Second is a 104 exhaust cam. Note the absence of the oil holes near the middle. Pretty much every other hole and pin matches though.

Third is the modified exhaust cam. I made a jig and added the oil feed holes for the cam advancer. During the drilling I thought for sure I would hit the plug sealing the end of the cam and have to re-do it. As luck would have it, it drilled to the center and cleared the plug.

[benzevo124]

Originally Posted by duxthe1

Ok here's some pics.



First is an intake cam. For the sake of accuracy this is actually the M111 cam that donated the 20 degree advancer. You can see the two holes near the center that feed oil to the advancer.



Second is a 104 exhaust cam. Note the absence of the oil holes near the middle. Pretty much every other hole and pin matches though.



Third is the modified exhaust cam. I made a jig and added the oil feed holes for the cam advancer. During the drilling I thought for sure I would hit the plug sealing the end of the cam and have to re-do it. As luck would have it, it drilled to the center and cleared the plug.

Nice job!
I'm also building a .980. I did some work on the ports. Interested to see how this will turn out. Are you planning on boosting or keeping it NA?


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[duxthe1]

I'm going to keep it NA for now. I'm curious to see what I can get out of it, and I've got my turbo wagon to keep me amused. I was planning to eventually turbo the red coupe before it got banged up. Now I've just got a lot of parts to swap to the blue one. This surprise head work just puts everything else further back on the list. I haven't even titled or registered it yet. Crisis / opportunity.

[MAG58]

What are your thoughts on the SSR on the intake side as the .980 port sits so low in the head?

Also do you have any concerns with the lifters pumping down at high RPM? I know I have started to look at maybe subaru lash caps to move to mechanical lifters as I've definitely experienced some pump down when the motor is at high RPM for an extended period.

Interesting with using the 20-22degree adjuster from the 111 or the 119. I wonder how that would play using some of the available aftermarket cams, EG CAT cams.

Do you plan to mess with the timing at all to get any more overlap? The M104 uses pretty rinky-dink cams and getting any more scavenge at high rpm would be nice.

[duxthe1]

The intake SSR doesn't look all that bad to me. As cast,the ports are pretty nice and without a flow bench I wouldn't touch it. The exhaust SSR looks like it could be better but I'm going to re-evaluate it when the head comes back clean from the machinist. It was too carboned up for me to tell if I want to do anything with it or not.

I intend to install the new cam with the opening event at pretty much the same time. I have to pass emissions with the car and a bunch of overlap could put that in jeopardy. I'm also a bit wary of valve to piston interference if you advance the intake much further, esp with how much my head is milled.

So I assembled the advancer on my exhaust cam this morning and taped over all but one oil hole. Fed air pressure though the remaining hole and actuated the plunger. Shazam! Functional cam advancer on an exhaust cam!

With the cam installed at the same opening event when advanced, and the 20 degree advancer installed, the closing event when retarded will only be 5-6 deg later than the stock intake cam and advancer combo. When its on the intake manifold's resonance it should be a beast.

[97 SL320]

There was a euro guy on this list that used a exhaust cam on the intake and a odd mix of SAAB pistons.

If you tell your machinist about the smaller cam base, he could leave the stems long , grind the seats / valves a bit harder and shim the springs. This could get you the 2 MM back.

I'm really interested in the cam adjuster swap. The AMG C36 M104 has a limited advance range and a more initial advance. I'm wondering if they used M111 parts?

I've also been looking at adding a adjuster to the exhaust cam, this would need a plate welded to the front cover to gain clearance.

[97 SL320]

My 104.980 has an idler gear below the intake cam. This may afford me an opportunity to correct the cam timing due to the head being milled. If I make a new spindle for this idler with an offset mounting hole I can rotate it to advance or retard the basic cam timing a few degrees.

I'd be real concerned about doing this on the tension side of the chain. If you go this route, be sure the tension wants to pull the idler in the direction of tightening the bolt not loosening it.

I'd be better with ovaling the sprocket / adjuster holes, rotating the sprocket to the next bolt hole and drilling a new dowel pin on the cam. There is a small chance you could rotate the adjuster on the helix to a new location and get the timing you are looking for. ( Or a combination of both )

[duxthe1]

Originally Posted by 97 SL320

be sure the tension wants to pull the idler in the direction of tightening the bolt not loosening it.

The original design already does this. The idler gear has an oil pressure fed bronze bushing that rides on a spindle that is 32mm od with a 24mm od stub that locates it in the head with a large 10mm thread bolt holding it. Its not likely to go anywhere.

I intend to time the intake cam with the offset spindle. The exhaust cam will get timed by slotting the cam gear bolt holes. I'm not keen on that approach with the intake cam for a couple reasons. First it is a PITA to disassemble and reassemble the cam advancer once, much less numerous times. Second, the holes that feed oil to the advancer line up with the oil feed holes in the cam. A little offset may be ok but I wouldn't want to stray that much. Only the 104.980 has this idler gear. Other 104 variants will not have this option, but for .980 engines it provides a simple, elegant adjustment.

[97 SL320]

Originally Posted by duxthe1

The original design already does this. The idler gear has an oil pressure fed bronze bushing that rides on a spindle that is 32mm od with a 24mm od stub that locates it in the head with a large 10mm thread bolt holding it. Its not likely to go anywhere.

Are you saying that the OE design is already offset?

If not offset, the original design works fine because any radial load applied to the sprocket just pushes radially on the bolt. If you offset the center hole of the idler, you are now producing a torque that will try to unscrew or tighten the bolt depending on what direction you are turning the bushing for adjustment.

Timing belt engines use this type of setup offset bushing on the slack side ( lower load ) and rotate it so any forces are trying to tighten the bolt not loosen it.

Mock something up with a loose bolt and give it a push by hand and you will see what I mean.

[duxthe1]

Its not offset, but any rotation of the stock part would only pull the threads in tighter. I get what you are saying though, I just don't think it will an issue. This part is solidly bolted into the head and at worst will only be a few mm off center. A quick plug in of some numbers into a bolt calculator buts the axial clamp force of its bolt well into the 10's of K- Newtons.

Speaking of, I finished the part this evening. It is made of cast iron, harvested from an old 617 camshaft. The part was turned to all critical dimensions except the finished diameter. It was then flame hardened and finish ground to the final diameter, followed by 3 stages of polishing. Turned out pretty well, pun intended

[97 SL320]

Yea, I know what you are getting at. I'm not talking about the sprocket rotation causing a loose bolt, I'm talking about chain tension acting on the now eccentric mount causing the bolt to loosen only under certain setup circumstances.

Here is a pic of what I'm talking about.

Also, timing chain loading is by no means smooth and is why the fewer cylinders / lobes per cam sprocket = lower chain life. As an example the M111 4 cylinder has timing chain wear issues where the same parts on a M104 inline 6 don't.