om616.912 rebuild dilemmas
 

Hello

I had a very embarrassing thing happen the other day.
I stripped most of my head bolts while unscrewing them. Please don't ask how :)
Fortunately they all came out but are not reusable.

The FSM says that starting February 1979 the head bolts were changed from hex to 12-point ones. The new ones are waisted and stretch when torquing. To account for the permanent stretch of the new type of bolts the bolt itself is 1mm shorter and the tapped holes in the crankcase have been lengthened.

My engine is the newer version after a power upgrade from 65 to 72 HP.
The problem is that it had hex head bolts

This doesn't make sense. It has the upgraded camshaft with longer stroke, chrome plated valve cap nuts, hardened lifters, hardened washers but the bolts are hex which would suggest that it's an older 65HP head.

Which bolts should I use? Hex or 12-point?
The FSM strictly says that on heads that came with hex bolts you should not use 12-point ones.

My concerns:
- The bolts changed in February 1979 but the power upgrade took place in August 1978 so I still could have the older hex bolts but an upgraded head (I think my engine number rules that out though - it's too high)
- My engine number is 616 912 10 093507 and this is way after the power upgrade of 1978. The upgraded engines start from 616 912 10 064618.
- Part websites say that the new 12-point bolts are for engines from 616 912 10 079811 - this would suggest i should use 12-point ones
- Head bolts might have been changed in the past from 12-point to hex by a butcher-mechanic
- Engine number might have been changed - probably starting to get paranoid :)
- I can't use the hex ones that came with it because I stripped most of them
- It is much easier to get the 12-point bolts (i have a set of goetze ones right now)
- I have not yet found anyone who sells the old hex bolts

Please help :)

Chap 05-105 is bloody confusing. It starts off by saying that engines should be repaired with the uprated head gaskets, the new washers, and the new bolts. But then it says to only replace the "older type bolt engines" with the older type bolts.

I can only guess to what has happened with your engine => I guess that the head was replaced at one point or other with an older head. The older bolts got carried over with the swap.

I can only assume that your block with the more modern engine number has the modification - if indeed the blocks were modified to accomodate the new bolts in some way - so you should be able to use the newer 12 point bolts and the new washers.

To be 100% sure I reckon you are best off asking whunter or mbdoc to be sure about this one.

I'm 95% sure it is OK - the chances of your engine number being faked are quite slim - no disrespect this is a W123 not a rare sports car once owned by Marvin Gaye...

...but if you think the engine number has been tampered with - post up a picture.


Get the correct tripple square socket for the new bolts if you decide to use them.

Marvin Gaye - he was most awesome! Good luck.

BTW, fasteners are fasterners, so as long as the length is correct, and the metal will allow the right stretch/TQ, run the triples. Ony trick to triples is to clean out the oil before you remove them later, as you can strip out the triple triangle and have to replace a few that way. I know. :confused:

I would do some serious measuring and if not satisfied carefully remove a very small amount off of the replacement bolts. Another option and preffered by me if felt it required might be to use thicker washers under the bolt heads. The thread length into the block should have enough of a safety factor in design to fudge it a little. Also clean and really check the block threads if the bolts are stripped.

It is not clear to me whether he stripped the heads of the bolts or the thread. If it is the thread i would have to look very hard at the threads in the block.

I have quite a few old bolts lying about. If you decide to go with second hand bolts I can check to see what is available. I may have both styles. If you buy them all new it will get expensive.

I stripped the bolt heads. Threads in the block are all ok. I am slowly convinced that 12-point bolts are the way to go. I will put the new goetze ones I have into the block without the head and see how much thread is in there. I suspect the block has some extra thread to accommodate for when parting surfaces of the block and head need refacing. I like the idea of thicker washers. The fsm says that the newer washers are hardened so I would have to find out what the hardness is if it turns out I have to buy hardness-equivalent thicker ones.

I have a 12-point bit set. It's called a 12 point spline set. The bits look like this

http://paceperformance.com/images/F31156613.jpg

so they have a rounded top.

Should I go with ones like this

http://paceperformance.com/images/F30130277.jpg

with a flat top?

Yes. I don't know from experience but I would estimate from general experience that there is about an extra quarter inch at the bottom of the head bolts.

With rounded bits you can sand/grind a bit off the end to 'sharpen' them

I'm not 100% sure about this but I think that the tripple squares are all flat top. Check with whunter, mbdoc or vstech on this one.

There is a 12 point star bit that is almost the same but it doesn't fit as well. As you've just had trouble with bits in the heads of bolts perhaps it is worth making the extra check?

I had to buy a set when I took my OM617 to bits - like a good little dog I just bought a set with tripple square written on the packet!

Now that I have the head bolts figured out thanks to you guys I can move forward with my rebuild.

Next curious thing is the piston diameter. They all have 91.46 stamped on them.
I measured all 4 with a micrometer (professional one) which I used before and always got accurate readings and they all measure 91.43mm.

What is this? STD pistons for this engine are 91.0mm or 90.9mm (2nd version cc lowered to 2399). I've seen oversize pistons and rings for 91.5mm and 91.7mm but what is this 91.46mm/91.43mm nonsense?

I measured at the thick end of course. The piston gets narrower towards the top to compensate for thermal expansion.

Could this be a custom piston?

I will get a tool and measure the bore diameter soon.

91.46 mm is the size of the over size pistons for the 616 and 617 engines.
Add 0.04 mm and you get the size that the cylinder should be machined to for that piston.
91.50mm is the size of the cylinder and 0,04mm is the clearance of the piston.

I used the new style head bolts on my 76 240 and never had a problem with them.

Are you sure? According to the fsm the difference between piston diameter and cylinder bore should be 0.029-0.038 mm for #1 cylinder and 0.019-0.028 for the rest. 0.04 mm seems way too much. The pistons are stamped 91.46 but actually are 91.42. STD pistons marked 90.9 are in fact that and no less than 90.88 again according to the fsm. Also there are no rings for 91.46 or 91.42 pistons. What rings would I use? 91.50?

Rings are sized for the cylinder, not for the piston.
From Mercedes Benz you can get, or at least could get, piston in about 0.01 mm increments to fit a given cylinder for repair purposes.
Over size pistons that come in a set of 4 or 5 direct from the manufacturer like Mahle, KS and Nüral are all the same size, and usually machined to fit a Cylinder that is honed to a round number like 91.50mm.
Rarely do you have to hone a cylinder to something like 91.49 or 91.51.
Most of the time the size of the cylinder will be printed on the label of the piston box and the needed clearance is stamped on the piston crown.
The FSM is just a guideline.
I personally would never give one of this pistons less then 0.035mm clearance and normally I give them 0.05mm.