m20 flywheel --> 13 lbs , Need more facts

[johnm01]

I want to lighten my single-mass m20 flywheel. Several R3v members have thrown out 13 lbs as the mass that they lightened to.

I modeled my flywheel in 3D CAD and have come to the determination that I will need to remove material from both sides of the flywheel in order to hit 13 lbs. There is some information out there that led me to believe that I could get down to 13 lbs with just a trim from the engine side.

To illustrate, below is a screenshot of an extreme amount of material removed from only back side. This is more than what photos of lightened m20 flywheels look like to me. CAD calculates that weight drops from 23.6 lb to 14.4 lb with this material removal.



Below is a screenshot of material removal from both sides, and this is what I believe people are actually doing based on my examination of lightened m20 flywheel photos. CAD calculates that weight drops from 23.6 lb to 13.7 lb with this material removal.



So let me cut to the chase-- Did you guys who cut your single mass m20 flywheel to 13 lbs have material removed from both sides? How much did you remove from the front? I have concerns because removing material from the front would increase the distance between the clutch fork and the pressure plate. And then wouldn't this necessitate the use of a longer release bearing (like the e21 bearing)? Not to mention this would complicate the machining job because flywheel screw holes and dowel pin holes would likely need to be re-drilled and/or the threads would need to be chased etc.

I want to know what you guys did exactly. Please shed some light.

Thanks,
John

[jeffnhiscars]

It seems to me rbe moving material from the friction face is going to throw your geometry out. FWIW I bought an 8lb fw from RHD for not much more than the cost of lightening a stock part

[johnm01]

Complicating matters: this is a single mass from a 325e. I read that the 325i flywheel is about 18lb (mine is nearly 24). I am putting this flywheel on an “I” engine. Do I need to use a clutch kit for an “e”? I am wondering if the”I” flywheel has different thickness from friction face to bolt hole face. If someone can point me to a thread that makes this clear I would appreciate it. I haven’t found a thread yet that fully clarifies this for me.

[Andre3127]

Complicating matters: this is a single mass from a 325e. I read that the 325i flywheel is about 18lb (mine is nearly 24). I am putting this flywheel on an “I” engine. Do I need to use a clutch kit for an “e”? I am wondering if the”I” flywheel has different thickness from friction face to bolt hole face. If someone can point me to a thread that makes this clear I would appreciate it. I haven’t found a thread yet that fully clarifies this for me.

Thanks for asking this, because I'm in your same boat. To complicate matters, it has come to my attention that two different bell housing depths were used on 325e transmissions.

[ak-]

Wish I can give a direct answer, but maybe I can help with a picture. Here is my stock e30 12.10lb flywheel.
I'm using stock components with a spec clutch without issue. 0 chatter.

[johnm01]

Thanks for asking this, because I'm in your same boat. To complicate matters, it has come to my attention that two different bell housing depths were used on 325e transmissions.

Yup! I have what I understand is the “second gen” eta Getrag 260 with the ears at the top to support the cast Aluminium shift carrier and it is the shallower bell housing (@ appx. 6.50 inches to slave cylinder reference face). The thread I learned this from says the ‘i’ Getrag 260 bellhousing is the same depth. So I was ready to pull trigger on a clutch kit for 325i. Now, unless someone tells me differently, I believe the ‘e’ flywheel is thicker and so I am going to research if the ‘e’ takes a different clutch kit to account for this flywheel thickness difference (perhaps a smaller release bearing?). I will report in this thread what I learn. But again, I would really appreciate an expert on this matter to weigh in.