This is for an 81 300SD but I do have two others with the same (917.xxx) engines and was wondering just how important this bypass pipe is. The reason I'm asking is, I cannot find the bonjoe(hollow) bolts to replace one that I heavy-handed and stripped out. Now I have a slightly bigger hole and threaded it with a M8x1.0 tap but the good bolt just slides into and out with slight drag. I will probably repair the hole that stripped and get a new bolt but I was just wondering about this in the meantime.

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1981 300SD

Would like to see the damage area if possible.

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I didn't take a pic of the area since it is a little hard to get my OLD digital camera there to take it along with my fat head to see what I was taking a picture of. The hole that is stripped is the one in the cylinder head on the bypass pipe from the block. It's not damaged too bad that I can't repair it but I was just wondering if I was to plug both the block and the head holes and discard the pipe if it would be detrimental to the engine and its performance. Also the hollow bolt for the pipe is what I heavy handed and just twisted it off.

I attached a pic of one of the other engines of the bolt and bypass pipe that I am talking about.

Attached Thumbnails

 

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1981 300SD

I may be wrong, but that looks like the oil line to the head.
If so, it is most definately needed!

To close, back off a couple inches please.
Hope it isn't my eyes going bad?
Fuzzy vision and no adult beverage! LOL

Hello,
You are refering to the coolant vent line, most are probably blocked solid anyway after a few years use, with the real risk of being branded a heretic, I think you can get away with blocking it off completely, but it will take a few days for all the air to bleed out of the cooling system(don't ask me how I know :p ).
The banjo bolts should be in stock at the dealer parts counter.
Hope that helps.

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Nachi11744

This is the water bypass pipe from the head to the block on the front of the engine.

Attached Thumbnails

 

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1981 300SD

Hello,
It is just a vent line from the water pump housing to the cylinder head water passage.

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Nachi11744

So if I plugged the block and the head to prevent water loss it wouldn't hurt it?!?!? I'm just asking because this is the only thing keeping me from starting this engine since I had my little fiasco a couple of weeks ago and I am waiting to see if I can acquire a new bolt to replace the one that I heavy handed and twisted off. I would of course only want to run the engine to check it out and until I could get the new bolt.

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1981 300SD

Hello,
I doubt the line is doing much to begin with...........all the cars I have worked on have had to clear it out with a stiff wire and sometimes replace it as the crud is set like concrete inside the small diameter steel tube.
When I replaced the waterpump on my M115 gas engine, I forgot to clear the vent line and had to top up the radiator every other morning as the coolant level kept dropping slowly. Could not find any leak, then remembered I had not cleaned out the little ba$t@rd.
The REAL bypass line is the water hose between the thermostat housing and the waterpump housing.
Hope that helps.

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Nachi11744

Jake,

I just about duplicated your case when I was taking apart the top of the engine to remove my cylinder head. It was the first time I had even noticed the "C" shaped tube and had now idea what it was for; but, I did know it was in my way and had to be removed. That was the lousy part. The banjo bolt to the cylinder head sheared while I was removing it. In the moment I failed to realize that it was not a regular solid bolt, and that it probably could not take much twisting, one way or the other. The only good thing is that I was in dismantle mode.

I tried like the dickens to use a couple of bolt extractor bits - no luck. The next of the sheared section only mushroomed out - did not turn at all. So, after dealing with the blown cylinder head, I got a HeliCoil kit No. 5542-8 (for M8x1 threaded bolts). I drilled out the remnants of the old bolt. And then proceeded to install the Helicoil as best I could. One obvious problem is that the insert is too long - longer than the depth of the hole in the casting. And unlike all examples of HC installation, this is not going into a hole that bottoms out. So, I guesstimated how much insert insert would actually be needed and then cut the insert, from the top, with wire snips - about 3 or 4 coils if I remember correctly. Space on the other side of the casting is so tight that I could not put my finger there to feel when the bottom of the insert reached the end of the hole. With about 4 coils beyond the end of the hole, then installation tool finished going forward. About 2 coils were remaining sticking out from the face of the hole so I used fine needle nose pliers to pull them and a coil inside the hole. I cut those off with the wire snips. Got a new banjo bolt from machinist. On reassembly, kappow! This time the bolt sheared when tightening it. Thankfully, this sheared bolt did come out using the bolt extractor bit - probably because the lack of corrosion to the insert hole. Back to machinist for another banjo bolt. He said that I should have used new copper washers. Back to reassembly. Got new copper washers - though I could not find ones as thin as the originals. Every thing seems to fit fine -- except for the fact that water leaks from the connection. Drip, drip, drip.

Did you have success in bolting up shut the block waterpump and cylinder head holes without leaks? Did you run the engine like that without any adverse results, like water overheating? If I can know that bolting up those holes with not do short term and long term damage I will do it today. Thanks for this thread.

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I always understood that the purpose of the by pass tube/hose is to allow coolant to flow through the engine while the thermostat is closed. Once the t-stat has opened the coolant flows through the radiator hoses and the radiator.

Bypass is maybe the wrong term, it's to bleed air out of the block etc.
(also holy thread revival record)

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