Headgasket 101 - where is TDC? - PeachParts Mercedes-Benz Forum

bgiovan · #1 01-01-2011, 04:29 PM

First time doing a head on my 617. Just want to triple check I am at TDC before removing cam. How do I know? Thanks!

moon161 · #2 01-01-2011, 04:45 PM

At #1 TDC, you should see the #1 cam lobes point up and form a V +/- 45 degrees from vertical

leathermang · #3 01-01-2011, 04:54 PM

UH... that is not exactly the proper way to locate TDC...

You need a dial indicator placed in line with the bore .. this will probably mean an extension to reach the top of the piston... then you turn the engine by ' hand' on the crank bolt...
and put a degree wheel ... which can be paper .. on the crank pulley.
Then you mark a place you know is before TDC on your degree wheel .. and go past TDC that same distance as measured by your dial indicator. Would not hurt to run it through the cycle more than once to confirm your readings...

Then you measure half way from the first and second marks... and that is TDC...

This method also will find if your crank pulley is rotated off what it should be. If TDC measured properly this way does not match your pulley Zero mark... need to look for the problem...

You have to find it by this ' non direct' method because the movement of the piston near TDC is just too little to be accurate.

bgiovan · #4 01-01-2011, 05:24 PM

Haynes Book Of Lies says I need to line up the notch on this outer washer:

with the mark on the backside:

.

I don't have any gauges (yet) and don't want to mess this up. Thoughts?

Thanks!

04 Diesel · #5 01-01-2011, 07:44 PM

Just make sure you are on the compresion stroke, lines lined up and lobes up and you should be fine.

bgiovan · #6 01-01-2011, 07:48 PM

Hate to sound like a total dunce 04 but can you tell this accountant turned diesel mechanic how all three things are correct? Compression Stroke? Lines lined up? Lobes up? Thanks!

vstech · #7 01-01-2011, 08:46 PM

if you are changing the head, the lobes will be wherever you put them, but the SIMPLEST method is to align the marks on the INNER washer... with the marks on the cam tower. if you do this before you pull the head, then put wireties on the cam chain gear holding the chain in place. you can drop the gear into the timing cover, then pull all the parts, and put it back together, and everything will remain aligned. as long as you don't move the crank during the process, it'll be fine.

bgiovan · #8 01-01-2011, 09:21 PM

Good deal John, thanks! Congrats on the Mod appointment also. Been lurking for a while and clearly you know your shyte! I don't live too far from Roy so if I mess it up bad enough I'm blessed to have local cavalry...

vstech · #9 01-01-2011, 10:04 PM

cool indeed! Roy is amazingly talented, but busy I'm sure! thanks for the vote of confidence! I'll do my best with the gig! I've been working on cars since I was 6 or so... nearly 40 years of car/truck work... my old auto shop boss wanted to have me take his shop over... at 16 there was no job he was afraid to give me... although I really worried him for a sec when a customer came in for a problem with his exhaust manifold, and he came in to see what I was doing, and found both heads out of the truck... he just asked me if I had it under control? I pulled the FE ford heads out, and showed him the complete rusted burned castings, and told him to ask the owner if he wanted new ford heads, or pulls from a yard 390/391... he never asked me if I had anything under control again...

bgiovan · #10 01-01-2011, 10:59 PM

Classic! Well, trying to take advantage of unusually warm weather (relatively) to get the head out of the donor car in my garage. Got the rockers off and the bolt that holds the plastic chain guide on. How do you get the pin out that the guide rests on? Neither the factory manual nor HBOL are clear.

vstech · #11 01-01-2011, 11:07 PM

a search will explain it in more detail, but you need to make a tool with long bolts, to screw into it and drive it out with more and more washers...

bgiovan · #12 01-01-2011, 11:10 PM

Odd. I was feeling around the other side (towards the firewall) and did not see an exit hole. HBOL did talk about that also but wasn't too clear. I'll search. Thanks John!

vstech · #13 01-01-2011, 11:15 PM

found one explanation for ya! the rail pin doesn't get pushed through, it gets pulled out lots of washers have to be put in over and over...
http://www.peachparts.com/shopforum/showthread.php?t=289220

bgiovan · #14 01-01-2011, 11:26 PM

Awesome! Thanks a bunch. Out of curiousity, been reading about cam differences in later years that give a little more HP. I'm pulling a known good head off a parts car that's been the bane of my wife's existence for the better part of a year (garage garbage collector) and putting it on the 79SD in my signature below that I just picked up from some old gent in Gainesville a few weeks ago.

He overheated the car pretty bad (so he says) and at a minumum the head gasket is shot (makes white smoke James Bond style). Figured taking this known good head off the '80 and dropping it into the '79 was the quickest fix to get it running (will be my daily). Then plan on slow boating a rebuild of the motor in the '80 to eventually drop into the '79.

Am I making more trouble than I need to consider putting one of the newer cams on now or should I just wait until the major tear-down? Probably just answered my own question!

Thanks!

leathermang · #15 01-01-2011, 11:33 PM

You guys are not actually determining TDC..
you guys are assuming certain things about the engine condition, the position of the cam ( in an old car at that ) that no cogs have been jumped or it assembled correctly..
I have told you the way to MEASURE TDC ACCURATELY.. which will show up any of those other things which might not be the way the Haynes manual assumed them to be.
Top Dead Center is by definition when the number one cylinder gets to the top of the stroke... you can not measure it directly because of potential play in the rod bearings and the piston and the minute movement it has when near TDC...
You are cam position and that does not necessarily have anything to do with TDC.. as vstech mentioned....
This is not something specific to Mercedes Diesels... this is how all engines are measured to find TDC...