Author Topic: Crankshaft Offset grinding (Read 8814 times)

RouteZeroDesign · « **on:** April 07, 2008, 08:05:10 AM »

Is it possible to increase the stroke of the m42 crank upto 85mm (from 81mm)
by offset grinding the rod journals?

The local machine shop said they could do offset grinding but im not sure how much you can get away with.

The reason im using the m42 crank (81mm) and not the m44 (83mm) is that the m42 crank is forged and the m44 is cast.

My goal would be for my stroke and bore to match that of the s42,
which is 85 stroke x 86.5 bore...giving close to 2000cc.

Any advice?

Boyracer · « **Reply #1 on:** April 07, 2008, 10:10:01 AM »

Are you going to rev the engine to S42 levels too (8500 rpm governed by rules)? Then I would be bit suspicious, altough I really am not an expert by no means!

To increase the stroke by 4 mm I think you need to decrease big end diameter by 8 mm?

And since stock big end diameter is something like 48 mm that is quite sizeable amount of material taken away, about 17% of diameter and 32% of area which is more important parameter. If strenght of crankshaft is limited by big ends and big end strenght is governed by diameter, you would lose 1/3rd of crank shaft strenght.

This is all assumptions made by me, anyone please correct me if I made an error

RouteZeroDesign · « **Reply #2 on:** April 07, 2008, 10:46:47 AM »

Yes, the idea is to rev the engine as high as possible. Im hoping for a 9000rpm redline so the rest of the engine will have to be built to match this.

The way you explain it makes sense, and removing material from the rod journals might not be the best idea, i just want to know how much can i get away with?

I wonder what crank the s42 uses....was it an offset ground m42 or m44 crank...was an inset ground m47...or was it a custom crank?

Boyracer · « **Reply #3 on:** April 07, 2008, 11:43:24 AM »

http://www.flickr.com/photos/25412802@N07/

You can be first to test this, just opened flick account

Took 4 photos from 1995 spec S42 documentation. Seems like quite stock crankshaft there but it cannot be perfectly stock because it has different stroke.

RouteZeroDesign · « **Reply #4 on:** April 07, 2008, 03:01:52 PM »

Okay i just did out the calculations.

First of all, the stroke is the distance between the center of the rod journal at tdc and the journal center at bdc.
On the m42 crank the distance between these two centers is 81mm.

If we want to increase this distance to 85mm,
it means we have to move this center outwards by 2mm.
So that the new distance is now 40.5mm + 2mm to the center line of the crank at tdc and the same again at bdc, making the stroke distance 85mm.

If the standard rod journal is 48mm in diameter, to increase the stroke to 85mm we need to move the journal's center outwards by 2mm. This will give us a rod journal of 44mm in diameter.

If the standard 48mm rod journal has an area of 1809.557 mm^2
Then the new 44mm rod journal has an area of 1520.530 mm^2
Which is a 15.97% reduction in area.

Im not sure if this reduction is acceptable, but only time will tell.

Thanks a million for those documents, they are brilliant.
it would be great if you could put up some more photos of the documentation, but i can understand your hesitance to do so.

kowalski · « **Reply #5 on:** April 07, 2008, 04:00:46 PM »

try going to the dealership with the pn for the s42 crank and see what they say:D

Boyracer · « **Reply #6 on:** April 07, 2008, 04:27:35 PM »

Ok so the loss of diameter/area is not that bad as I thought, that is a good thing.

I have hundreds of pages of that documentation so I cannot put them all to web because it is huge amount of work and it might be also illegal due copyright issues. But if you have really pressing need for something I think I can snap few more photos

By the way, there was a link on this forum to a BMW document from year 2003 that listed lots of Motorsport parts and their numbers and prices. If I recall right, there was few S42 cranks listed, might be still available but they were thousands of euros.

nuvolarossa · « **Reply #7 on:** April 07, 2008, 04:39:51 PM »

first thanks boyracer for those infos!

now I have a question as I know nothing when speaking of engine building:
-escluding price and difficulty to found 320d M47 cranks, there are advantages with this solution versus M47 crank, that will give 88mm stroke and it's forged too?

How much the difference in weight between the two will affect revs?

from ETK:
M42 crank weight: 14.5 kg
M47 crank weight: 16.6 kg
---------------------------
difference : 2.1 kg

In that document there are two versions of S42 crankshafts: the 1995 version is listed at 2172,99€ and the 1997 version is 5777,60€.... why all this difference?

RouteZeroDesign · « **Reply #8 on:** April 07, 2008, 05:43:17 PM »

Kowalski-
I think they would probably show me the door but you never know. The main BMW dealer for Ireland is Joe Duffy motors where my dad was today picking up his new old car. (new to him, but old in terms of years)
Anyway, my dad was talking to one of the mechanics there who has an e30 m3, which had a turbocharged s52 engine in it :eek::eek:
So you never know, they might be a bit more enthusiastic there than most garages.

BoyRacer-
Yeah the reduction in csa isnt that huge but i wonder what effects it has on the dynamic balance of the crank. Obviously il need to have the crank balanced after the offset grinding and other machine work but il probably have to speak to somebody about the dynamic balance, and if moving the journal center point outwards will affect the counter weights.

I might have to ask you to post up a few more in the future but its cool for the moment. Where did you say you purchased the documents? Was it german ebay?

I have the bmw motorsport etk, is that the one your talking about? I think it was nuvolarossa that posted it.

nuvolarossa-
Im not sure exactly how it would affect the revs but i cant imagine it would help. An extra 2.1 kg of rotation mass is a huge amount when your trying to spin it at 9000rpm.
My goal would be to create as light a bottom end as possible.

As far as the difference in price goes, im really not sure.
Maybe one is a modefied m42 crank and the other is a custom crank, but im only guessing at that.

cecotto · « **Reply #9 on:** April 07, 2008, 06:51:08 PM »

..forget it...

Just reread the thread, and realised i read it wrong.

.deleted.

D. Clay · « **Reply #10 on:** April 08, 2008, 01:21:02 AM »

Cranks are also stroked by grinding and welding stellite or a similar hard alloy on the journals and then reground. You can add up to 3/8" of stroke this way. It eliminates finding smaller bearings and the danger of going through the hard outer layer of the journal. I don't know the cost these days or how it would compare with a M47 crank costwise. There's also the thing of keeping the bore happy with the stroke for the intended use - torque, high end, or low end power.

swiss318is · « **Reply #11 on:** April 08, 2008, 03:55:45 AM »

my friends!!!... the original s42 crank is sth around 3500.-$ :-) i am looking for a m47 crank to do a s42 replica... i think thats the easiest an strongest version..

RouteZeroDesign · « **Reply #12 on:** April 08, 2008, 02:27:16 PM »

m47, being a diesel crank is very strong, but its also 2.1kg heavier.

I guess il have to wait and see. If i was building this engine for racing i could afford the expence of trying both, but i think il just stick to my intuition and go for the lighter forged crank...i guess i can be the guinea pig

D.Clay- Thanks for the information, il have to look into that further before getting mine done.

As far as the intended use for the engine, the idea is to match the characteristics (roughly) of the s42, so high rpm power is where i will be aiming for.
The idea is to keep the stroke relatively short, and by using a tried and tested spec that bmw motorsport has already used.

The problem with going small bore, long stroke and high rpm is that piston velocity becomes an issue.
The e46 m3 is an example of this, where it had a relatively long stroke yet revved to over 8000 rpm. It was said to me that the piston velocitys at this rpm were similar to that of an f1 engine.

kowalski · « **Reply #13 on:** April 08, 2008, 02:30:41 PM »

Quote from: nuvolarossa;46351

first thanks boyracer for those infos!

now I have a question as I know nothing when speaking of engine building:
-escluding price and difficulty to found 320d M47 cranks, there are advantages with this solution versus M47 crank, that will give 88mm stroke and it's forged too?

How much the difference in weight between the two will affect revs?

from ETK:
M42 crank weight: 14.5 kg
M47 crank weight: 16.6 kg
---------------------------
difference : 2.1 kg

In that document there are two versions of S42 crankshafts: the 1995 version is listed at 2172,99� and the 1997 version is 5777,60�.... why all this difference?

I plan on lightening the m47 crank to make up for that difference.

RouteZeroDesign · « **Reply #14 on:** April 08, 2008, 02:44:42 PM »

Can i ask what bore and stroke youre aiming for?
The only reason im going for 85mm stroke is because i want to keep the engine under 2000cc, for insurance and for racing category.
if i were to go for a longer stroke then the m47 crank would be a good option.

Just wondering, in what way can the crankshaft be lightened?
I know about blading the counterweights, but i assumed this was to make the cranks journey through the oil easier.
Where else can weight be cut off?