water pump to head pipe

[Ron]

In assembling the engine from boxes of parts, I'm now the to point of applying that little pipe between the head and water pump.  The pipe has 2 banjo fittings.  I'm assuming this thing bleeds air from the water pump housing on coolant refill.  There already is a bypass hose under the thermostat, so this is probably not a bypass.  Problem is that the threaded hole in the head is too big!  This head is a replacement head, applied by the PO.  It looks like the banjo bolts are 12mm and that size bolt passes into the head hole barely touching.  I see several choices:

1.  Plug both holes and bleed pump whenever there's a fill, just pulling the plug.
2.  Enlarge banjo fitting hole on pipe and put in next size bolt (if I can find the proper one for that size hole in the head).  This would weaken the banjo fitting.
3.  helicoil?  It would have a very small cross section.

Any opinions?

Ron

[waqas]

I would go with the helicoil solution. (or similar fitting)

Have you measured the thread size of the hole already there?

[Dave Gallon]

1) the hollow bolt is 9 mm. Are you referring to the distance across the hex flats? This measures 12 mm on the part in my hand. We do not make reference to the distance across the flats when specifying the diameter of a bolt.
2) I just checked the parts literature and do not see any reference to a larger hollow bolt, even for use on the 280SE sedan after the 280SL was out of production.
3) any chance you have not identified the correct hole in the cylinder head? What is the diameter and thread pitch of the hold in the cylinder head? To be certain, you should probably find a bolt that threads into the hole smoothly and then measure the bolt.
4) if you can provide the cylinder head hole size, I may be able to match a hollow bolt to it.
5) would you please post the markings on the driver's side of the cylinder head?

[Ron]

OK, I think I'll try a helicoil, but conditions are not perfect for such a solution.  Please see below for Dave's Qs:

1) the hollow bolt is 9 mm. Are you referring to the distance across the hex flats? This measures 12 mm on the part in my hand. We do not make reference to the distance across the flats when specifying the diameter of a bolt.

I took it into the NAPA dealer today (Sunday).  It turned out to be 8M-1, they had helicoils for 8M-1.25, but can order the proper one.


2) I just checked the parts literature and do not see any reference to a larger hollow bolt, even for use on the 280SE sedan after the 280SL was out of production.

Yes, I have one random gas 6 head, for a 280S, plus two /8 cars, a 280 and 250, all have the M8 bolt, so I don't think M-B changed this for this head.


3) any chance you have not identified the correct hole in the cylinder head? What is the diameter and thread pitch of the hold in the cylinder head? To be certain, you should probably find a bolt that threads into the hole smoothly and then measure the bolt.

Last night I discovered this problem, today in better light/more time, I see the threads in the hole are shallow and have signs of J-B Weld (my engine seems to have several applications of that stuff   )  I think the threads in there are worthless.


4) if you can provide the cylinder head hole size, I may be able to match a hollow bolt to it.

Please see my comments below.


5) would you please post the markings on the driver's side of the cylinder head?

It is a: 127 010 50 20, which I was told was for '65-'67 230SL and '61-'67 220S.

Comments on hole size.  The NAPA store loaned me the helicoil M8-1.25 kit.  It says use a 21/64  (inch) drill for the hole.  Here at the house, I have 19/64, 5/16, and 23/64.  Using just the bit in my hand, the first goes into the hole, no binding.  The second, 20/64, has to be rotated to get through the hole, so there must be a burr in there.  The third will not enter the hole at all.  Hopefully the hole has enough "meat" that the proper 21-64 will make a smooth wall for the tap.

My thoughts are to order the kit, and buy the proper bit, and give it a go.  However, I hate enlarging the hole.

Only other solution, is to take off the thermostat housing, with a longer M8 hollow bolt, insert bolt and screw M8 nut inside head to bolt.  I could not find hollow bolts today.


Ron

[ja17]

Hello,
These by pass fittings were only one size.  You have damaged threads in the head. A helicoil is the best solution so you can restore it and use the original banjo fittings.

If all else fails you can plug the openings. The by-pass only aids bleeding the air out of the system when changing coolant. It does not work very well in the first place.

Try the heli-coil first. 

[Benz Dr.]

The bigest problem I see with these small banjo bolts is that they break off in the head or pump and then have to be drilled out. I really doubt that it stripped out but nothing is impossible when it comes to the strange things people do to these cars. A more likely answer is that someone drilled the hole out too big.

The fix Joe describes is your best bet. I've had to do this repair before and I can tell you it's not easy. The coil is very fine and looks more like a spring but once it's installed it will be a good repair. Use anti seize on the threads of the banjo bolts before you install them.

Fixing this in any other fashion will tend to induce leaks that will drive you crazy. MB doesn't give advice nor do they tell you how to do repairs like this. The work shop manuals only tell you how to do the repairs that they mandate. Anything outside of this is stuff that puts you on your own and a little bit of expirience can go a long ways.

NOTE:  If you aren't comfortable doingh this repair get a machinist to do it for you. It might look easy enough but believe me it's something that can go wrong. The hole has to be drilled straight and the cylindr head is valuable. If all else fails, the hole can be welded shut then a new hole the proper size can be drilled and tapped. The head would likely need to come off for that repair though.

[Dave Gallon]

To repeat and emphasize Dan's comment:

Use anti-sieze compound on the hollow bolts!!

In fact, always use anti-sieze compound whenever you have a steel fastener threaded into an aluminum part. One of the most important locations are the bolts that secure the thermostat housing cover.

[Benz Dr.]

(WOOOO HOOOOOO something we agree on ! )

 Oh, yeah. You want to use more anti seize than lock tight on these cars.


   I asked my machinist and yes, you would need to remove the head should any welding need to be done. The best method of all but some work required.

[Ron]

Going with all the advice, I installed (my first) helicoil.  It looks great, and the banjo bolt, with anti-seize, is snug in it's new helicoil home in the head.  Thanks for all your comments.

Ron