900 Top end swap for a 650

Anyone done it here? Just curious, since I saw quite a bit of mention of it on the 650 Turbo forum. I am happy with my 650 as it is, but the idea of more power always gets me thinking...

 

I'll take that as a "no"...

 

If you don't also swap in the 900 crank, but just do pistons, jugs, and head you end up with around a 750. I'm not sure how rod length or compression height works out on this mod, but the pistons could end up being "short". If you swap everything, it works.

 

deep in chacal's parts list he talks about swapping cams, i don't know if it works or not

 

It's not a good idea to be honest. If you're not changing the crank you end up with a slightly lower compression in the heads which will result less power after all. On the other hand the distances between the inlet ports are quite different so you can't use the original carbs.

If you want more power, then use the jugs and pistons from a 900 or from a 750. More displacement, higher compression ... exactly what you're after.

 

I have been thinking about the swap on my 750. It sounds like the parts are all physically interchangeable for the 3 engine. (from what I am reading) What about the cam & crank? And are the gearing in the engines different? I haven't been able to find the 'blueprint' specs of the engines. (ie. gear ratios, bore, stroke, cam specs, etc...)

I am looking for an old 750 Midnight engine to put on my Seca, but I want to build it up before the swap. (I just like the black engines)

 

Well, AFAIK the crank wont be a problem. But the cams ... they are different. The 900 cams are much wider and the bearing set up is different. You can't use the 900 cam's in a 750 - 650 engine and vice-versa. On top of this, the cam lift and timings are different.

I'm not sure about the gear ratios (someone suggested the final ratio is higher in the 900 engine), but the heads are quite different. Larger valves, larger inlet - exhaust ports, different port spacing, different rubbers and carbs. Of course, the pistons, jugs and the rings are different as well, however you can swap them into a 750 engine (650 should be fine as well) without any problem. The bottom end is quite similar except some minor differences (like five spring clutch vs. six spring clutch).

 

In my police special it had XJ900 standard pistons- 68.5mm, std 750 is 65mm. There is plenty of meat to bore your 748cm2 jugs out- i found a set of wisco 67.5mm pistons and a standard set of jugs/barrels to machine out.

I was lead to believe that the police special was fitted with XJ900 cams, sad to hear it may not be so.



A fellow in Brisbane was selling a hack on eBay last year and claimed XJ650 turbo cases with XJ900 crank and XJ900 top end got him around, he started with the 650 and when a rebuild was needed, put in the XJ900 gear so i hear it works.

Suppose if you wanted a bullet proof bottom end you can't go past the XJ650T cases.

Unless the case mounts are different from the 650 to the 900, then why not save the hassle and shoehorn in a eBay acquired XJ900 motor into your pony, sure it would be nice- but think of your family man..

 

interesting, I didn't know the police ever used seca's.