M20 Stroker Build: Take 2

[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.

[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.

[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.

[redlightpete]

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

[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.

[digger]

Rods will probably be fine but I personally wouldn’t use the rods like that due to the possibility of fatigue at the stress concentrations. Over time the material accumulates fatigue damage. That area sees some tensile loading under the compressive cylinder pressure

[Lugnuts]

Thanks for the responses everyone :)

Took all 6 rods to the local machine shop. Big ends were lightly honed...just to clean up the surfaces and take the high spots off rod 4. You can't feel anything raised now.

Measured the big end ID: 90 degrees from the parting line, then +45 diag, -45 diag. Caps tightened to 15 ft/lb + 45 degrees. Bentley says 70 degrees, but I'd like to avoid stretching the bolts. Figure this should be sufficient for measuring.

Small end bushes were checked by the machine shop. He said they were fine for the new pins and didn't need replacing. Checking them (accurately) might be an issue since I only have telescopic bore gauges in that size...

Big end spec as per Bentley is 1.8898 - 1.8901 (red), 1.8901 - 1.8904 (blue)...not sure what the red/blue classification means - there's no marks or anything on the rods. Crank classification maybe?

Rod Number	Measurement
1	1.8900"
	1.8898"
	1.8900"
2	1.8900"
	1.8899"
	1.8900"
3	1.8900"
	1.8899"
	1.8899"
4	1.8901"
	1.8901"
	1.8900"
5	1.8900"
	1.8899"
	1.8901"
6	1.8900"
	1.8900"
	1.8899"

Hopefully my mic and bore gauge calibration is good. Getting that "feel" for measurements is definitely a learned skill, especially down at the 0.001/0.0001" end of the scale!




Also weighed the rods with bolts, no bearings (kitchen scales, so...not super accurate).

1	563g
2	563g
3	564g
4	564g
5	564g
6	563g

[Lugnuts]

Rods will probably be fine but I personally wouldn’t use the rods like that due to the possibility of fatigue at the stress concentrations. Over time the material accumulates fatigue damage. That area sees some tensile loading under the compressive cylinder pressure

Hmmmmmmmm. Maybe I should find an additional set of M52 rods....just to swap out number 4 with a non-scratched alternative.

Damnit digger, why you gotta put that voice in the back of my head lol :D

[digger]

sorry, rods are important and they aren't expensive you'll sleep easier. Given this is a take 2 build you dont want to end up with take 3. i'm sure if you were just building a slapper you'd just run them

[zaq123]

sorry, rods are important and they aren't expensive you'll sleep easier. Given this is a take 2 build you dont want to end up with take 3. i'm sure if you were just building a slapper you'd just run them

exactly. rods are hardest working components of the engine. Considering all that time and expenses that go into the build, they are 100% "do it right" item. It's nowhere near a category where people are skipping to check P to H, V to P and other clearances (almost no cash required) and later really really regret it. But still...........use the best parts you can afford for your project.
My approach : would I buy this part in such condition for my intended build? No...and there is a reason for it. Why would I use it then? Just because I already have it and it's "free"?

[Lugnuts]

sorry, rods are important and they aren't expensive you'll sleep easier. Given this is a take 2 build you dont want to end up with take 3. i'm sure if you were just building a slapper you'd just run them

No need to apologise - the sanity check is part of the reason I post here. Plenty of collective wisdom that would probably have saved me from the errors I made during Take 1 - had I taken the time to ask/advertise my build.

On the subject of BMW rods, M54B30 appear to be compatible? 135mm, and going by the bearing shell and bushing sizes, also 48mm big end, 22mm pin. Only difference I can make out is the rod bolts are 3mm longer vs M52.

[ForcedFirebird]

m54 have snapped rod caps which aren't service-able. I would suggest sticking with the m/s52 and earlier.

[ForcedFirebird]

Actually, after thinking about the last reply, your machinist can repair that rod easily. He can take the cap off, grind the mating face, then re-bore the big end. That's how rods are reconditioned, anyways.

[Lugnuts]

m54 have snapped rod caps which aren't service-able. I would suggest sticking with the m/s52 and earlier.

As best I can make out, there appears to be some overlap with late model M52 rods and early model M54 rods (P/N 11241437210). Looking at a couple of sets now and they appear similar to my current rods - with uncracked parting lines. Definitely going to confirm though, so thanks for pointing that out.

Speaking of serviceable. Finding replacement rod cap dowels (P/N 11241280833 7,5X11,2), which I assume would be damaged when removed for cap resizing seems next to impossible. You'd think such a simple item would be available anywhere/everywhere. All the other block dowels are easy to find...

[Andre3127]

As best I can make out, there appears to be some overlap with late model M52 rods and early model M54 rods (P/N 11241437210). Looking at a couple of sets now and they appear similar to my current rods - with uncracked parting lines. Definitely going to confirm though, so thanks for pointing that out.

Speaking of serviceable. Finding replacement rod cap dowels (P/N 11241280833 7,5X11,2), which I assume would be damaged when removed for cap resizing seems next to impossible. You'd think such a simple item would be available anywhere/everywhere. All the other block dowels are easy to find...

Not hard to find, but takes a minute to ship.

www.ecstuning.com/b-genuine-...l/11241280833/