There is some info on different pre-chamber variations in the engine manual for 617A, I think.  Do you have that book?  I can check if you like, but can't guarantee a speedy reply as I'm stupid-busy at work now with not much time for G stuff in the evenings.  But I'll try and remember to take a look.

You are talking about pre-chambers for the 617A in both cases, right?  The 617 ones are for sure a lot different from the 617A.  To be sure you get the right ones, you should know the engine number, or better yet, the VIN of the car the engine (or head) came out of.

-Dave G.

PS - this brings up a good question for you diesel guys..... If someone drops a bare head in your hands, how do you tell whether it's for a 617 or for a 617A?  If the head has pre-chambers in it, I think you can look at the hole pattern in the end of the pre-chambers poking out the bottom of the head.  But if it's just a machined casting, how can you tell which engine it's for?

Sorry to say, Steve, what you have there is a 617 head.

I re-read your note after pulling up the page I'm attaching below. This is a page from a really cool manual that I borrowed from my mechanic when I first got the 617A motor. MB was so keen on the 617A when it came out that they published a whole special manual that just detailed all the differences between the 617 and 617A.  One of the biggies is the pre-chamber design.  As you can see from the 2nd pargraph at the top of column 2, "...the diameter of the prechamber neck has been increased to 16mm..."  In this I think we have our answer to my previous question.  617 and 617A heads can be distinguished by their prechamber neck clearance hole diameter.  14mm for 617, 16mm for 617A.

Additionally, there may be another clue.  See the prechamber pix at the bottom left of the first picture below?  One way that the 617A manual says one can distinguish 617 from 617A prechambers is by the fact that the 617A ones have a smaller hole for the glow plug - about 9mm - where the hole for glow plug on 617 is much larger.  If you look at the two pictures in the SECOND attachment below, showing cut-away views of the prechamber area, it looks like heads that took the pre-chamber with the larger glow plug hole, would also have a correspondingly larger hole bored in the head for the glow plug as well as a different shape to that hole.

So you can probably confirm by looking at the glow plug hole (is it about 9mm like a 617A, or much larger, like 617) area of the head, but from your description of the prechamber neck hole diameter, it looks like you have a 617 head on your hands.

The 617A manual also makes NO mention of different pre-chambers fitted to 617As, only that the 617 chambers must not be used in 617A heads.  Every other place that differnt variant parts were fitted to different vintages of motor, the manual DOES make distinct note of the potential variations.  So I think we can be sure that only one type of prechamber was fitted to 617As - the one with 16mm neck.  This is the rael hard evidence that the head you have isn't originally off a 617A.

The remaining question for you is of course, can the head you have be safely modified to accept the larger prechamber.  I'm afraid I can't comment on that one.  The fact that the book is adamant that the 617A type prechamber "can not" be installed in the 617 (617.912) engine head means of course that they don't fit, but what it might imply about potential for mods to the 617.912 head (which it seems you have), I can't infer.  My only comment is based on general mechanical point of view that boring away 2mm worth of iron in that area of high stress seems risky. 

Note also that in the second paragraph at the top of the FIRST column in the first attachment below that the prechamber design changes were undertaken due to the higher thermal stress present in the higher performance turbodiesel engine.  This tells me that fitting the 617 prechambers into your 617 head to use it would not work well, and could potentially even cause catastrophic failure if the prechamber, bing not up to the task of handling higher thermal stress, were to fail, crack, chip, or break apart inside the engine.  It sounds to me like fitting the head you found with the old style prechambers is not an attractive option either.

Wish I had better news.

-Dave G.

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prechambers.jpg (68KB - 6 downloads)

glowplug.jpg (36KB - 4 downloads)