As I have previously posted, I purchased a rebuilt cylinder head from Cylinder Heads International (CHI) in Grand Prairie, Texas. I know I took a chance, and I think there is a problem, but I haven't installed it yet and I will return it if necessary!!

My understanding from CHI is that the head has been pressure tested, magnafluxed etc....and that a valve job has been performed to MBZ specs. I called for more info. and the guy who works on the MBZ heads said that valves are not new, they are used valves that have been reground. Also, new guides, seals and springs were installed. I asked about angles of grinding, and he said that "a three angle valve job" was performed.

Anyway, here is the problem: I have the full set of MBZ shop manuals. In the section on machining valve seats (05-291), it gives the proper specs. for reground seats and new valves. One dimension that I checked was the recess distance of the valve flat surface from the mating surface of the head. I measured using a metal straight edge ruler layed on edge on the mating surface of the head, and measuring recess of the valves using metric feeler guages.

The MBZ manual says that the MAXIMUM distance for both exhaust and intake valves is 1 mm. On the head I recieved, the intake valves are recessed 0.7 mm below the mating surface of the head (within spec), whereas the exhaust valves are recessed 1.2-1.3 mm from the mating surface of the head!!! This is .2-.3 mm greater than the 1.0 mm specified in the MBZ manual.

Should I return the head?? Note that the MBZ specs. are for NEW valves and reground seats. The valves in mine are not new...they were reground...is this therefore OK?? Is it common to just regrind old valves, or should a "valve job" always involve new valves.

I dont really know what I'm doing, but I want to come up to speed before I call them.

I also removed retaining nuts and springs to feel play in guides (I didnt pull threaded portion of valve stem into seal). There is absolutely no play (wobble) within the valve guide. Valve stem seals are obviously new, and rotocaps rotate freely.

Any advice will be greatly appreciated.

Thanks,


Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)

I have always gotten new exhaust valves and have the intake valves reground. The additional clearance would impact your compression slightly. The exhaust valves might have been overground or done mutiple times. Call them and ask them why there is a discrepency. It sounds like new exhaust valves should hav been installed.

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63 190d (sold)
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87 300SDL 135k
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Can't comment on the tolerances but typically a valve job involves cleaning and re-lapping the existing valves. New valves are generally only used to replace burnt or otherwise damaged valves.

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Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.

I assume the loss of compression would be due to the slightly larger volume of the combustion chamber. Given that pressure decreases proportionately with increase in volume, I would guess that the compression would be decreased by about 0.0002%....is this correct, or am I missing something.

Is there some other critical problem having to do with less mass (thinner) on the valve itself, or limitations on adjusting valve clearances??

While I would rather have new exhaust valves rather than re-ground ones, I dont know if I really need to return the head for something better. I would like to get another 250,000 miles out of the car, as everything else has been maintained in tip top shape and restored. The only consequence of having re-ground exhaust valves may be that the next time I need a valve job, I wont be able to re-grind them....I will then need new ones. However, as long as I adjust valve clearances, valves shouldn't burn and I should get another 250K out of the car without more head work. Is this correct?

Anyone out there agree? Disagree?


Thanks to all for lots of help,

Mark

__________________
1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)

Mark:

The M-B engine manual has a section discussing refacing/grinding valves - both intake and exhaust - for your turbo 300D (see page 05-280). New intake valves have a width (I called this the valve margin, M-B calls it spec "h" for the distance from the edge of the valve that contacts the valve seat to the face of the valve) of 2.54 mm and the exhaust width of 2.49 mm. When both are refaced the minimum width is 2.00 mm. Your spec for the recess allowed for the valves is for new valves contacting new seats of a maximum 1.00 mm. If M-B allows the valves to be refaced to 2.00 mm with a loss of about .5mm from the width, then a recess of 1.2 to 1.3 mm for your exhaust valves does not seem to be excessive - especially since a refacing job on the valves also means M-B anticipated the seats would also be ground to clean them up for the refaced valves, then a lap job to finish the contact surfaces between the valve and seat - meaning a slightly greater recession of the valve into the head. I would think your head is OK to use with the dimensions you gave. As a comparison the non-turbo 300D has a maximum recess allowed of 1.5 mm for new valves and new seats, but no spec for refaced valves or ground seats - strange to not have a spec for this, but the M-B manuals have other inconsistencies as well.

I would check to see if you can get the correct valve to rocker arm to camshaft clearance. On the bench assemble the head (valves, springs, rotocaps, lock nuts, cap nuts, rocker arms, cam bearings/towers, camshaft, etc., etc.) and check to see if you can adjust the valve clearances for a cylinder or two since the valves will be closer to the cam than before. This would be easiest for cylinders one and two, but you can do all five cylinders if you want. See what I'm driving at?

Keep a close look on valve clearances and adjusting them more frequently will ensure they do not get too loose which means the cam lobe will let the valve strike the valve seat a bit hard. Living in Boston means it gets colder than other parts of the US and M-B has a different valve clearance spec when it is colder which you need to consider (at -4 F. add .0019 in.) if you should get this cold - let's hope not. The valve rotocaps will help a lot with ensuring even wear to the contact surfaces of the valves and seats.

You're right, any compression loss would be insignificant and should not be a worry - in my opinion.

Good Luck!
Tom

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America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
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I will go read that section in the MBZ manual! Sounds like this head is OK. From my experience, I believe there is a wide range in the cap-nut adjustment so I dont think there would be any problem getting the proper clearance, even with the valve seats and valves ground. If I do have a problem, I could grind a little off the top of the valve stem, allowing the cap nut to screw in further (I dont think the sodium core is up in the stem)...I really dont think this is a problem, but I will do as you suggest...set up on bench first before installing the head.

Again,

Thanks very much

Mark

__________________
1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)

You can get the slide hammer portion of the prechamber puller aparatus from AUTOZONE. You only have to leave a deposit equal to the value of the tool. When you return the tool you get your deposit back, all of it! so they lend you the tool for free! I don't know if there is a time limit but there are quite a few of there stores in Mass so it shouldn't be hard to cover a couple day period. You should probably call and find out if you can reserve it for when you need it or if you can keep it awhile get it as soon as you can. Just bring your adapter to check how it will mount. They've got valve tools also spring compresors and others, if you need a torque wrench they've got that, although that might not be as accurate as you want, oh well. Good luck!