M20 Stroker Build: Take 2

[ForcedFirebird]

get some plastigauge (ACL flexi gauge is available locally) to double check what the mic's are telling you

Good catch. Haven't realized the bearing haven't been bolted down yet. Bearings change shape once clamped.

[digger]

get some plastigauge (ACL flexi gauge is available locally) to double check what the mic's are telling you

[Lugnuts]

Thanks Firebird.

After measuring and remeasuring (and re-remeasuring!) the #6 clearance, the 2 sets of Kolbenschmidt mains are coming up too loose (0.075 / 0.0029"), and the BMW greens are just too tight for my liking (0.045 / 0.0017").

Based on the crank journal, the sweet spot is 60.035 / 2.3635...which the other mains come in at (or acceptably close to).

The set of ACL Race bearings I was sold push the clearance even further out of spec...0.085 / 0.0033".

Measured the main tunnels w/o bearings, caps torqued to 44 ft/lb - all pretty consistent (~65.015). Bearing shell thicknesses ranges from 2.503 (BMW green) to 2.493 (KS set 1) to 2.480 (ACL)...

Mixing bearing shell halves (green/yellow) is a no-go, since the other 6 mains are right on the money.

So, I'm thinking I might have a bad set KS mains (both sets were bought together a while ago). Put an order in for a pair of BMW std/yellow late model thrust bearings (cheaper than a full set from either Glyco, KS, or King)...fingers crossed!

[ForcedFirebird]

.0018" is the high side of the factory tolerance - which is a little tight IMHO. Rule of thumb when building engines is .0001" for every 1" of journal diameter. .0026" is a little loose, even by "rule of thumb" standard (mains are 2.3"). This is the very reason I feel the s54 had rod bearing issues. They left the bearing clearance around .001" but increased the crank pin by a fair bit (and were already "tight" on the 1.7" pins).

I'd go with the .047mm/.0018"

[Lugnuts]

Going back and forth over a bearing clearance on the #6 main journal.

Choice of 0.047mm / 0.0018", or 0.067mm / 0.0026".

Should I go tighter or looser?

[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/

[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...

[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.

[ForcedFirebird]

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

[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.

[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"?

[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

[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

[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

[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