8bolt, 8 counterweight, forged, 71mm short throw crank.

[mttpekin]

Hello.

 

I've been looking and can't really find a definitive answer. 

 

The item in question: 8bolt, 8 counterweight, forged, 71mm short throw crank.

 

I've been told these exist and would be out of an early small displacement car eg 1602. 

 

Does anyone have one in hand as proof they are real? I'd love to see pics/markings. I've heard stuff like 'yeah those were the ones that used to always go in the trash' and 'everyone upgraded to a 2l back in the day' as to why they're not around/available/findable.

 

If you have one/two/ten that you don't want, please let me know.

 

Thanks 

 

Matt 

 

 

Edited March 30, 2022 by mttpekin
clarity

[uai]

never heard of anything like that in an M10.

don't know about M12

[tzei]

Maybe it’s 72.6 mm stroke? Then S14B20 crank.

[mttpekin]

 

This is the most relevant thread I can find but still not super definitive.

 

I am going to have a flywheel made for my application so I guess 6 bolt would work too.. 

 

Thanks,

Matt 

[Mike Self]

Didn't the 1502s that were built right at the end of 02 production have essentially 1600cc engines, which would have had the short throw crank; and didn't all the later cars (after about March '69 production and including 1600s) have 8 bolt crankshafts?

 

mike

[Milo]

There is no such thing as an 8 counterweight 71mm 1600 crankshaft.

 

The short stroke 71mm M10 crankshaft was first used in the NK 1500 in 1962. It is a forged crank and BMW continued using it in the NK1600 and later the 1600-2 or 1602. The 200mm clutch components of the 1500 were also adopted by the 1600-2, 1602, NK1800 & 1800ti. During the later part of 69 BMW switched from 6 bolt to 8 bolt cranks on all their M10 motors including short stroke motors.

 

This 71mm short stroke crank was also adopted by the 1802 and very late NK 1800. This version of the 1800cc motor was redesigned to incorporate the short stroke 71mm crank because the short stroke motors are smoother. The earlier versions of the 1800cc motors have a long 80mm stroke (like the 2.0L), but with the small 84mm bore (like the 1600). In 1969 BMW switched this around to a large 89mm bore and short 71mm stroke on their 1800cc M10 motors. This configuration of the 1.8 was also used in the e21 and e30 1.8 variants. However, during the late 70s BMW stopped forging all their cranks. The 1.8L short stroke crank of the e21 and e30 is therefore not forged. So, the earlier 71mm forged crank of the 1500, 1600, 1600-2, 1602, 1600ti, late 1.8L NK 1800 and 1802 is a better crank since it is forged. Otherwise, they are interchangeable. Some folks think of the smaller displacement  short stroke M10s were not built to the same quality as the larger displacement motors. And, this is simply not the case. They pick up a 1980s or 1990s vintage 71mm crank, confusing it with the earlier 71mm crank, and claim that 71mm cranks are not forged. Like I said, BMW stopped forging all their cranks by about 1979. I think the "M" cars were the only ones receiving a forged crank.

 

The 71mm short stroke cranks are not fully counter weighed because they don't need to be. They only added the extra weights on the long 80mm stroke 2.0L due to the extra mass of the pistons and clutch. Adding weight to your crank slows down your acceleration and revs. The 71mm short stroke forged crank is probably the best and longest lasting M10 crank as short stroke motors wear slower. The connecting rod bearings don't get beat up as much and the pistons speeds are lower. And, the motors are silk smooth. There is a good reason they kept producing the short stroke 71mm M10 motors into the 90s.

 

 

Edited March 31, 2022 by Milo

[Mike Self]

4 minutes ago, Milo said:

During the later part of 69 BMW switched from 6 bolt to 8 bolt cranks on all their M10 motors including short stroke motors.

Don't know the switchover engine numbers for Euro cars or for US 1600s, but for US 2002s it was at 1665200, same time the factory switched to diaphragm clutches from the older style coil spring pressure plates.  This was sometime in early March; my '69 was assembled 399 cars before the changeover.

 

mike

 

[mttpekin]

@Milo thank you.

 

I now know what I am looking for and will cease searching for non existent parts.

 

Matt 

[Milo]

29 minutes ago, Mike Self said:

Don't know the switchover engine numbers for Euro cars or for US 1600s, but for US 2002s it was at 1665200, same time the factory switched to diaphragm clutches from the older style coil spring pressure plates.  This was sometime in early March; my '69 was assembled 399 cars before the changeover.

 

mike

It was at some point during 1969. I don't know when either. Unlike the 2.0L motors which switched to the diaphragm clutches at the same time they switched to the 8 bolt crank, on the 1600 they kept using the same 200mm clutch as first fitted to the NK 1500 in 62-63. However, BMW eventually fitted the 215mm diaphragm clutch to the 1.6L and 1.8L motors starting with the e21 316 in 75-76. For those of you who have an 8 bolt 71mm crank in their 1600, this is the cheaper and alternative clutch to use compared to the 200mm clutch which has gotten really expensive.

[NickVyse]

I've never heard or seen a 71mm m10 crank that had more than the 4 counter weights. And not sure why you'd want them, with the shorter stroke and smaller displacement they're butter smooth right up to the red line and the lack of weight obviously helps them spin up quick.

 

Like someone else said there's always the 2 litre s14 option

[mttpekin]

Success is easy when you know Carl and actually know what you're looking for..

 

parting lines indicate forged, a little trig & a caliper validate stamping marks. and its 8bolt! we're in business. 

 

Thank you for the help

 

 

[TobyB]

Yeah, the distortion in the lines looks like it's been whanged with a heavy die.

 

So you never said-

 

why do you want this torqueless wonder?

 

 

t

 

[NickVyse]

1 hour ago, TobyB said:

Yeah, the distortion in the lines looks like it's been whanged with a heavy die.

 

So you never said-

 

why do you want this torqueless wonder?

 

 

t

 

 

I'm looking forward to building a tweaked motor with a 71mm crank for my 1600-2. They have such a great sewing machine like nature, at least the current old engine in the 1600-2 does and as far as I know it's totally stock (never opened it up).

 

It has surprised a number of 2002 owners, and as long as you keep the revs up you'll never have a stock 2 litre get away from you on a twisty road. Obviously the 4.1:1 helps as well as the early car weight.

Edited April 3, 2022 by NickVyse

[Harv]

Nick, maybe you should use an S14 block with the largest pistons too.  
 

john

[2002iii]

I was thinking slap that crank in a 2.0l bock to make a 1.8l and add a turbo!!!!!