Topic: Will a 1981 650 'nighthawk' overhead and carb fit my 78 cb550k?

[Tretnine]

All I really want to know is if the 1981 650 'nighthawk' will fit my 78 550. I know you can use 650 overheads from early 80s models, but I'm not so sure what the 'nighthawk' was in that time frame. I thought actual nighthawks started in 82.

[bryanj]

sohc maybe
dohc doubt it

what is an overhaed?

[Tretnine]

Sorry, the cylinder head, cam, and cam chain. I'm looking into the 650 top end on a 550k mod. I may also have a line on a 79, but I want to know what actually works. Also curious about what carburetors would work on which bikes.

[cb650]

Nitehawk came out in 82.  What does the frame say?  They will also have the CV carbs which wont "bolt on" to the 550 like the 79/80 meck carbs will.
Nitehawks also had one big rectangular gauge housing.

[Tretnine]

so if I wind up with nighthawk top end I would have to switch to fruity square gauges to have the tach read correctly and the carburetors wouldn't work. They won't 'bolt on' but they could be jimmied? I'm hoping to get a 79 motor and carb rack, but we'll see...

[mlinder]

If you use a 650 head, you'll need to use 650 pistons. Asked this of Paulages a while back. Running 550 pistons in a 650 head will make compression VERY low.
Also, it isn't a direct bolt on.

You can use the 650 cam (assuming it's a sohc 650), but you'll have to attach the 550's cam gear and use the 550 cam chain.

[Slippery]

Nitehawks also had one big rectangular gauge housing.

'83-'85 DOHCs did, but '82s had the standard round clocks.

[jeepster]

Indeed:

Nitehawks also had one big rectangular gauge housing.

'83-'85 DOHCs did, but '82s had the standard round clocks.

[Tretnine]

If you use a 650 head, you'll need to use 650 pistons. Asked this of Paulages a while back. Running 550 pistons in a 650 head will make compression VERY low.
Also, it isn't a direct bolt on.

You can use the 650 cam (assuming it's a sohc 650), but you'll have to attach the 550's cam gear and use the 550 cam chain.

Tell me more of this. Do I need to overbore the engine or is the difference in stroke length? Why is compression so low with 550 pistons?

[cb650]

Yes the "gauges" them selves are round but take them off and the whole rectangular (mostly) comes off as one piece.  Not like the round individual of the other 650's.
I only mentioned that as a way for him to ID the bike he had. 
And yes if you use a 650 cam you will need a 650 tack. Off any sohc650.

[mlinder]

If you use a 650 head, you'll need to use 650 pistons. Asked this of Paulages a while back. Running 550 pistons in a 650 head will make compression VERY low.
Also, it isn't a direct bolt on.

You can use the 650 cam (assuming it's a sohc 650), but you'll have to attach the 550's cam gear and use the 550 cam chain.

Tell me more of this. Do I need to overbore the engine or is the difference in stroke length? Why is compression so low with 550 pistons?

You'd need to bore. The combustion chamber of the head of the 650 is larger. The 550 pistons are near flat while with 650 pistons are domed.

[Tretnine]

If you use a 650 head, you'll need to use 650 pistons. Asked this of Paulages a while back. Running 550 pistons in a 650 head will make compression VERY low.
Also, it isn't a direct bolt on.

You can use the 650 cam (assuming it's a sohc 650), but you'll have to attach the 550's cam gear and use the 550 cam chain.

Tell me more of this. Do I need to overbore the engine or is the difference in stroke length? Why is compression so low with 550 pistons?

You'd need to bore. The combustion chamber of the head of the 650 is larger. The 550 pistons are near flat while with 650 pistons are domed.

Interesting. After I first read about this mod I thought it'd be simpler than it is. There are really a lot of things needed to make this work right. At any rate, it still seems like something I'd be interested in, provided I could get the parts without paying too much. I might have a line on an engine, but how much is an overbore like this? Does anyone have a good referral for somewhere to get machining done in central VA?

[mlinder]

650 bore and stroke : 59.8 x 55.8 mm

550 bore and stroke : 58.5 x 50.6 mm


Not a huge overbore.

Most people just slap on the 650 cam to the 550 for a mellow little boost.
The 650 head, from what I've seen, looks vastly superior in terms of low and design, though, so I think It would be a good mod.
According to Paulages, there some fairly minor machining that would need to be dont to get the 650 head to fit on the 550 jugs, though.

[Slippery]

Yes the "gauges" them selves are round but take them off and the whole rectangular (mostly) comes off as one piece.  Not like the round individual of the other 650's.
I only mentioned that as a way for him to ID the bike he had. 
And yes if you use a 650 cam you will need a 650 tack. Off any sohc650.

Ah, outer gauges with the warning lamps between, in the same panel. Gotcha.