engine and drivetrain One oil hole or two?

[harold]

I am ready to start re-assembling my engine.  The small end bearings in the connecting rods do not need to be replaced, since there is negligible wear.  There are what look like two oil holes in the rod small end, one vertical hole, facing the top of the piston, and a second angled upward from the forward face of the rod.  Only the top oil hole is drilled through the bushing, and is chamfered nicely.  The second "oil" hole just dead ends. 

 

Should this second oil hole be drilled through?  Obviously it has been working fine like it is.  The shop manual only makes vague reference to the oil holes, just saying to drill them after replacing the small end bushing. 

 

I also asked this question of the owner of the machine shop where I had all the machining done on the block and head.  He said that most engines have only the vertical oil hole.  The second lower one on the M10 rods being placed at the area of highest bearing pressure didn't seem like a good idea to be drilled through. 

 

I couldn't find anything via multiple searches on the FAQ.

 

The engine history is unknown to me.  The block has no serial number in the usual place, just two X's on either end of that pad.  There are some stamps at the top back of the block, one looking like a BMW roundel (about 1/4" diameter), a "21" and a few other random numbers.  The pistons in it were already 0.25mm oversize, and looked like standard compression piano tops.  Mfg markings on the pistons were not the "M" for Mahle,  but they were clearly German in origin.  Three of the connecting rods had the typical 1-1, 3-3, 4-4 marks on one side of the big end. The one in position 2 had manual metal engraver markings only, marked "29".  The odd thing is the positions were reversed - rod #1 was in cylinder 4, etc.  It had good compression and ran fine before I took it apart. 

 

H

[TobyB]

Hmm- I don't remember a hole in the side- the standard way of doing things is as your builder suggests.

And should be just fine- it even works ok for race motors.

 

And your description of the engine sounds like a factory rebuilt unit- the .25 pistons, especially.  If they rebuilt

a previously- rebuilt engine, then it's quite possible that the rod marks come from BEFORE the factory rebuild.

Rods are stupidly durable in stock motors, so they may have reused them a second time.

KS would be the other usual German KolbenSchmidt, or PistonSmith...

 

t

 

[Tommy]

I do remember seeing those extra side holes. I think it was an e30 316 engine so I guess it's late model rods that have those. I can't remember if they were drilled through the bushing, I would guess they were. But I have no idea why they were added to the design.

 

  Tommy

[harold]

Here is a clip from the BMW shop manual.  The oil hole seems to have been around a while. 

 

The text states: Note when fitting: The oil hole in the connecting rod small end bushing must face in the same direction as the arrow on the piston.

[kbmb02]

1 hour ago, harold said:

Here is a clip from the BMW shop manual.  The oil hole seems to have been around a while. 

 

The text states: Note when fitting: The oil hole in the connecting rod small end bushing must face in the same direction as the arrow on the piston.

 

 

Many of the OEM rods I've seen have that side oiling hole .... so, who knows the precise reason the factory wants the hole facing forward? -KB

[TobyB]

wasn't there just a pic of someone showing skirt scuffing on the front edge of the bore?

 

t