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Aside from the obvious extra plug hole in the head, what are the differences between the two engines? Can the single spark be fitted into the later bike with just a cylinder head swap?

As I understand it the Twin Spark is there to make the engine more efficient i.e. more Bang for your buck = little more power and better consumption.

I'm also given to understand that the second plug fires milliseconds after the first plug and the two explosions rush towards each other to ensure that all the petrol air mixture is burnt = efficient.

that being so the ECU must tell the stick coils to fire at different time to each other (per cylinder) therefore I think if you changed the cylinder head - or Just bunged up the second plug hole or in fact just disconnect the second plug you would also need to change the ECU to one from a single spark.

You could just trying a quick run with the second plugs disconnected to see the difference it would make.

However why would you want to make the engine less powerful and less efficient?

I think you've got the wrong end of my stick. I don't want to fit the older engine into my newer bike as-is, I want to know if the only difference is in the cylinder heads.

I have an '02 bike with light damage but because the subframe lug is broken they have given it a cat b which is a bit annoying since I had already bought a mint body kit of eBay but there you go. The engine only has 42k miles on it whereas my '04 bike is at 60k and rising.

What I want to know is if it's worth holding on to the early engine as a spare for my later bike? If the only difference is the heads then I would say yes otherwise I'll probably sell it.

Wrong end of the coil stick apparently - my bad

I think ECU would still need to be kept as well as the lump just in case the programming is different

Pete. wrote: What I want to know is if it's worth holding on to the early engine as a spare for my later bike? If the only difference is the heads then I would say yes otherwise I'll probably sell it.

Afaik mechanically the short block is the same , and pistons and cylinders are the same confirmed by using this website

http://www.maxbmwmotorcycles.com/fiche/PartsFiche.aspx

Definitely worth holding on to imo

The obvious. Cylinder head, rocker cover and spark plug cover, extra coil stick. Not so obvious. The ECU (Motronic unit), the wiring for the plugs, HT coil (under the tank), ignition coil. Also be aware that those bikes without heated grips runs a lower output alternator.

A look here will put you at ease.
Linky: http://www.realoem.com/bmw/enUS/partgrp ... 22%2C0432_

Hope it helps.

Thanks for the replies yes it's the short block I am thinking of. I'm weighing up the pros and cons right now of either selling the lump complete and keeping it in case I need the block. Changing the heads is no problem for me.

To add a degree of complication I don't know how much longer I will even be able to keep using my '04 bike into London because they don't seem to be able to make their mind up when the new ULEZ levvy for bikes will come in. If it's next year it's hardly worthwhile keeping it because I'll have to buy a '07 or newer bike anyway or start paying congestion.

The final option is to source a frame and put all my stuff on it but that will cost quite a bit too as I'd need to buy other bits too, and then of course I'd have two bikes that can't go into London when the new laws come in.

or don't go into London

Not so easy when you work there every day...