M20 Stroker Build: Take 2

[ForcedFirebird]

That’s potentially just an amount of porosity that BMW thought was acceptable.

Doubt it. There's probably 100-120 BMW OE rods at my shop, and they are quite well engineered. Many people don't know BMW actually have stamps in the bottom of each rod, containing numbers such as "A1, B7, B5" etc. Rods were matched in sets, and are within +/- 5gm for each individual engine. Rods can vary somewhere in the 30-60g range from heaviest to lightest across the grades. When building engines at work, they are balanced each end to .2gm for a particular engine to go out the door.

[redlightpete]

That’s potentially just an amount of porosity that BMW thought was acceptable.

[ForcedFirebird]

If the spots are recessed and not protruding, the rods are usable, albeit a spot for potential micro cracks under high stress applications.

Many years ago, in less than ideal conditions, I sanded the rod journal on a spun rod bearing, filed the rod through the oil pan, and then continued to run boost through the motor - eventually failing another rod bearing since I was new to tuning and turbo builds. Not many people were running twin (or any) turbos in their v6 f-bodies in the early/mid 2000's lol. Take that with a grain of salt, but that's my take.

[Andre3127]

I had some dents in the big end of one s50b30 rod that I used in my M20 rebuild (forged and lighter, instead of stock cast rods). I used a small oilstone to remove the high spots around the dents and assembled it normally. I haven't boosted the engine and only have a few miles on it, but I've had no issues so far. No spun bearings or anything. You can always have the rod caps ground and bores honed if you're really worried.

[Lugnuts]

Building another M20 to replace the one that's currently in my E30. I built that one too, but I nerfed the compression ratio by using flat top pistons with an 885 head (NA engine). The pistons also sit slightly below the deck - maybe 1-1.5mm. Basically I messed up the important clearances when it comes to getting decent performance.

The car runs fine and doesn't smoke, but it feels a tad lacklustre when you press the pedal. Couple different dyno runs put the performance pretty much at stock.

Nothing crazy in terms of parts...just want to get the engine running well with the expected modest bump in performance. New pistons are IE Mahle Super Strokers. Crank and rods are used M52/135mm. Should be 10:1 when all said and done.

Installed cylinder head already has an IE 272 with HD rockers. Intending to reuse those parts. Engine's only done about 35,000k over the last 5 years. Got another head at the shop now being cleaned up: exhaust side guides, seats and new exhaust valves cut.

Block, pistons, crank, rods, flywheel (lightened single mass) are back from the shop after clean up and machine work.

Plan is to build as much of the long block as possible before doing a (quick) long weekend engine swap.

So anyway, figured since it's the weekend, I'd start cleaning parts for reassembly. Discover this on the big end of rod 4:



Kinda pissed because I spent a fair chunk of cash having parts machined/balanced and checked. No one said anything about scratches on one of the rods...

Not sure if this is a show stopper for an engine that will live a pretty relaxed NA life, and if I should be looking for a replacement rod. Can't really get any nails in the scratches, but you can feel them when running a finger over.

Going to keep cleaning parts and take this to the (different) machine shop that's working on the cylinder head, see what they think.