Stroking/boring M20B25/B27: Sectioned blocks show critical dimensions. PICS!

[tinkerputzer]

my buddy Jared made a build thread for his 3.1 liter m20, here is some pictures from his build thread.

Thanks Alex. Diggers findings pretty much confirm there are no differences in oil gallery locations between the b25 and b27 blocks. A light chamfer on the bottom of the bore is really all that is needed to clear the rod shoulder with the bigger cranks.

[digger]

they were available right back to late 1981. I know the 525e from 83 till about 85 could be found with 11:1 comp ratio, then they changed to 10.2:1. Reference codes for the blocks changed, digging into my files, this will tell you exactly what block you have and where it came from originally.



All blocks should have the identifier located on the flat section below the distributor drive (or where it would have been on later blocks). It's that rectangle shaped flat section. Should find the block identifier and the engine number. First 3 numbers/letters above (example 276KB 05 A) is the new revision identifier, some blocks will have it, some wont. Next group is the original identifier used when they were produced (example, 202702 A). Last group is the actual bmw part number (example 11 00 1735340 E), often won't find it anywhere, usually used for cross referencing with parts catalogue.

edit: Added more blocks. If the block doesn't have the flat plate, i think ive seen it sometimes stamped below the dipstick.

was restricting myself to e30's since i was told it is from an e30.

i dont see any numbers on the block other than the engine number and

1278921

[iwantspeed]

my buddy Jared made a build thread for his 3.1 liter m20, here is some pictures from his build thread.

In regards to clearancing the block to fit the s52 crank. Apparently there are some internal differences such as smaller oil gallery casting between the E and I blocks. The early model (86-87) e blocks being more desirable for fitting oversided cranks. That being said the only clearance issues I ran into while test fitting the s52 crank in my e block (with s52 rod and stock piston) was the #1 con rod big end would just barely clear the aux shaft every couple revolutions. This was due to the raised text on the aux shaft. The other issue was when the crank is swinging through bdc the shoulder of the con rod just kisses the bottom of the bore where the casting line left a bit of extra casting flash behind. It probably wouldn’t be an issue after the block was bored but it was something I wanted to address anyway. Here are some pics illustrating the issues.

















High tech tool used to measure bottom of bore to conrod shoulder clearance. A piece of banding strap about .025”











All bottom end clearances were checked with a dial bore gauge prior to bottom end assembly. The goal was to adhere to the .001” of clearance per inch of journal standard. Ended up using kolbenschmidt main bearings due to glycos being all over the place clearance wise. Tried using kolbenschmidt rod bearing but couldn’t get the clearances to where I wanted them. Had to use clevite std and +1 shells to dial the clearances in.


Assembly time:

[88SuperETABD]

I guess i misread your post. I thought you were saying eta's have that one 2.7 block and all i's had the late b25 block, which in my case is not the case.
You were talking about the early 2.7 block tho i assume

[Madhatter]

they were available right back to late 1981. I know the 525e from 83 till about 85 could be found with 11:1 comp ratio, then they changed to 10.2:1. Reference codes for the blocks changed, digging into my files, this will tell you exactly what block you have and where it came from originally.

256E1 01 A 325I A 11 00 1730858 M20 B25
256E1 02 A 325IALL A 11 00 1730864 M20 B25
256E2 01 A 325I/1 A 11 00 1730889 M20 B25
256E2 02 A 325IX A 11 00 1730866 M20 B25
256EX 01 A 325IMNV A 11 00 1730887 M20 B25
256EX 02 A 325ALLMNV A 11 00 1717163 E M20 B25
256K1 01 A 202501 A 11 00 1730862 M20 B25
256K1 02 A 202502 A 11 00 1730856 M20 B25
256K1 03 A 202503 A 11 00 1730871 M20 B25
256K1 04 A 202504 A 11 00 1730872 M20 B25
256K1 05 A 202505 A 11 00 1730835 M20 B25
256K1 06 A 202506 A 11 00 1730838 M20 B25
256K1 07 A 202507 A 11 00 1730806 M20 B25
256K1 08 A 202508 A 11 00 1730836 M20 B25
276EA 01 A 525E A 11 00 1286389 E M20 B27
276EA 02 A 525E/1 A 11 00 1289057 E M20 B27
276EB 01 A 525E/2 A 11 00 1735080 M20 B27
276KA 01 A 525E KAT A 11 00 1735076 M20 B27
276KA 02 A 27ETAUS A 11 00 1278827 M20 B27
276KA 03 A 27/1ETAUS A 11 00 1735062 M20 B27
276KA 04 A 325ETAUS/1 A 11 00 1735064 M20 B27
276KA 05 A 325ETAUS A 11 00 1735071 M20 B27
276KA 06 A 325ETUS/1 A 11 00 1735339 E M20 B27
276KB 01 A 525E KAT/1 A 11 00 1735077 M20 B27
276KB 02 A 528E KAT A 11 00 1735063 M20 B27
276KB 03 A 11 00 9059131 E M20 B27
276KB 04 A 325EKAT A 11 00 1735072 E M20 B27
276KB 05 A 202702 A 11 00 1735340 E M20 B27

All blocks should have the identifier located on the flat section below the distributor drive (or where it would have been on later blocks). It's that rectangle shaped flat section. Should find the block identifier and the engine number. First 3 numbers/letters above (example 276KB 05 A) is the new revision identifier, some blocks will have it, some wont. Next group is the original identifier used when they were produced (example, 202702 A). Last group is the actual bmw part number (example 11 00 1735340 E), often won't find it anywhere, usually used for cross referencing with parts catalogue.

edit: Added more blocks. If the block doesn't have the flat plate, i think ive seen it sometimes stamped below the dipstick.

[digger]

I can add to this, although, I have contradicting info. I personally pulled this m20b27 out of a 87 325es and had my block bored to 85mm. My block looks like your pictured b25 block. Mine clearly doesn't have the 2.7 stamped. odd. mine was indeed a eta crank with 130 mm rods...maybe there is just late vs early blocks? And the cut off is somewhere near 87 when they switched I am guessing.


And here is a picture of a 85 mm bore on a block. To show meat in between cylinders (Sorry, not cutting my block up :))

I'll update with measurements in between.

Heres the crank side.

And I will also be using the 84mm stroke m52 crank. I weighed it on my bathroom scale and it came in at 45.8 lbs. And thats naked, no bolts or nothing.

whats contradicting?

mine in an early eta block with Welch plug on the back eta were available in 86 and 87 i dont know the year of the engine that the engine came from i suspect it an 86

you can get it ultrasonically tested but i doubt it is neccesary

[Madhatter]

what's different? the casting for the immediate shaft?

Your crank comes within a few hundred grams of the weight of digger's crank.

[88SuperETABD]

I can add to this, although, I have contradicting info. I personally pulled this m20b27 out of a 87 325es and had my block bored to 85mm. My block looks like your pictured b25 block. Mine clearly doesn't have the 2.7 stamped. odd. mine was indeed a eta crank with 130 mm rods...maybe there is just late vs early blocks? And the cut off is somewhere near 87 when they switched I am guessing.


And here is a picture of a 85 mm bore on a block. To show meat in between cylinders (Sorry, not cutting my block up :))

I'll update with measurements in between.

Heres the crank side.


And I will also be using the 84mm stroke m52 crank. I weighed it on my bathroom scale and it came in at 45.8 lbs. And thats naked, no bolts or nothing.

[digger]

here is the issue with big counterweights.

they will hit the bottom of the piston.

i machined off the entire skirt and shown is a 135 mm rod which would work but stick out of the block.

if you used a 130 mm rod it would put the piston close to the top of the block but it would foul the pin boss. you do would need to machine the crank counterweights to get this crank to work as per a fellow members 2.8L build.

by my calculations with a 89.6 mm crank the relative position would be unchanged from shown. the 89.6 mm crank has 2.5 mm smaller counterweight but has 2.8 mm bigger throw so much the same

there are 84mm cranks in oz and UK which have smaller counterweights dont know about the US (this crank came from a US car)

[digger]

here is where the rod gets close to the bore.

note i have slightly chamfered the bottom of the bore but the stock bore comes with a sharp corner.

i also profiled this rod so a stock rod is alot closer and the M52/S52 rod is slightly closer.

if you use the big 89.6mm crank you lose 2.8, plus you lose about 0.5 to 1mm ith a S52 rod compared to what i have shown so if the bore is not chamfered there is a light interference





i will attempt to do more photos with a un modified 135mm stock B25 rod and maybe a 130mm B27 one.

i dont have a M52/S52 stock rod only a "special" one with 21mm pin size as opposed to 22mm and as a consequence i have no junk piston to match

[digger]

84mm crank with stock rod profiled to match an aftermarket Molnar rod

there is about 3mm clearance to the raised lettering on the shaft.

add in a 89.6mm crank instead and you lose 2.8mm clearance (5.6mm stroke but you lose only half in the throw). hence MM sell a machined intermediate shaft which does little more than clean up the surface.

note that clearance problem is in the width at the big end not where i have ground material away. i have a 24V M52/S52 style rod and it is the same width. Aftermarket rods are often wider here for strength and stiffness so take note with a big crank and aftermarket rod

[digger]

some more details on crank clearancing

[LJ851]

Thread title updated to be representative of all content.

[digger]

yeah. There would be 1kg weight in the hub, the sprocket, bolt and the trigger wheel. what did MM say they weigh?

24kg

[Madhatter]

yeah. There would be 1kg weight in the hub, the sprocket, bolt and the trigger wheel. what did MM say they weigh?