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Old 03-12-2013, 08:48 PM
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Join Date: Jul 2011
Posts: 526
Conrod numbering m117

I am taking the pistons out of my engine for a re-ring job and I noticed that each conrod has a handwritten number on the bottom corresponding to the cylinder number it belongs to.
Is this factory, or has this engine been apart before?

Also, I was looking on my recently acquired spare (m117) engine, and the pistons only come to about 5mm of the block deck. Is this normal?
I am used the pistons coming pretty much flush with the block.


Last edited by vandor; 03-12-2013 at 10:49 PM.
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Old 03-13-2013, 10:16 PM
Tony H's Avatar
Join Date: Oct 2005
Location: Bandon, Oregon
Posts: 1,050
I have dissasembled a few engines and have not noticed any numbers other than the size stamped into the piston. On the M117.981 to 984 the manual shows the piston down the cylinder from 3.15 to 3.60mm. I guess that is how they lowered the compression ratio. On the later .985 and .986 they used dished pistons to lower the CR. Are you going to have your cylinders power honed? If you use a flex hone there is no way to get the cylinder round and remove taper. These engines have very tight tolorances-between .0008" and .0012" for the cylinder-piston clearance. If the cylinder is not perfectly round the rings may not seat.
Tony H
111 280SE 3.5 Coupe
Manual transmission

Past cars:
Porsche 914 2.0
'64 Jaguar XKE Roadster
'57 Oval Window VW
'71 Toyota Hilux-Dad bought new
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Old 03-14-2013, 09:54 PM
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Join Date: Jul 2011
Posts: 526
Hi Tony,

Thanks for the info. Actually I honed the cylinders a few hours ago. The bores looked really good, I could see the crosshatching even on the thrust side. With any luck they are not oval or barrel shaped. I suppose I could check clearance at different heights and angles to see.
The top rings is cast iron, not chrome like the ones I took off, so they should be easier to seat than chrome rings.

'72 280SEL 4.5
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Old 03-15-2013, 02:37 PM
Join Date: Sep 2005
Posts: 664
I rebuilt my 1971 6.3 engine and found the same numbers on the connecting rods. They looked like they were etched in with an electric pencil.

That's because they were. The engine was balanced and each piston was balanced with the connecting rods for the maximum smoothness out of the V-8.

Yes, this cost a lot, but with high horsepower engines it is a wise thing to do. Back in the early 70's the only way to balance an engine was to take it apart, balance all the parts, and rebuild it. I had this done to a Chevy 427 in the late 60's and the cost was around $600. I don't know that it gave the car anymore HP, but it made what HP there was more useable.
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