Topic: cb650 head gasket work for a cb550?

[Soos]

Am curious if anyone knows if using a cb650 head gasket will work on a cb550 bored out using cb650 pistons?

I know the cylinder spacingh is (nearly?) identical, but what about the oil holes, and the stud hole patterns?


Thnx for any info.

l8r

[Sam Green Racing]

The 650 pistons are only .3 of a mm bigger than the 550s maximum re bore size. you will probably get away with the stock 550 gasket unless anyone knows any different.

Sam.

[paulages]

the cutout for the cam chain tensioner is different on the 650. i don't think that should be critical, but don't quote me on that one. the majority of it is the same, but the slots where the tensioner rest in the cylinder body is different. i can tell you that the locating dowels, oil passageways, and bore centers line up perfectly.

here, i think i have pictures (these are of the base gasket though):
550:

this is 550 cases modifies to fit the 650 tensioner. the top slots are the 550, and the lower ones were milled:


i believe the top side of the 650 cylinders are also differently shaped, but i'd have to look next time i'm at my shop. i'll try to remember to do so.

[Soos]

Awesome guys, thnx for the info!!


Hope this helps anyone who is doing a cb550 overbore with cb650 pistons!!

While i'm at it... can anyone conform 100% whether the deck height is EXACTLY the same on the 550 and 650 pistons?

From what I remember reading they are close enough that it is negligible... but I don't have a set of 550 pistons to compare to.


l8r

P.S. Hey Moderator(s) - maybe this should be in the tips and tricks area, or a FAQ area?

[paulages]

soos- i'll measure up all these things when i go to my shop this afternoon. i have every part of a 550 and 650 engine in bins beneath my bench. when you say deck height, do you mean pin height (distance from the pin center to the shoulder of the piston)? have you mocked up the 650 piston yet? it is domed quite a bit, and the 550 pistons are flat. i imagine there will be a huge increase in the compression ratio. maybe this is a know mod and i should shut up? 

[Soos]

You are correct assuming I am meaning centerline of the pin to the shoulder of the piston when I mention deck height.

And from the pics of 550 pistons I would imagine it would bump the compression a bit, but i would not imagine so far as to not be able to rum pump gas anyways...

I'm asking mainly 'cause as I overbore my 650 engines, I would love to see these still usable 650 pistons put to better use than scrap metal.

Hey paulages... you should see the 750 domes i'm putting in my 650... they make the 650 pistons almost look flat.


l8r

[paulages]

Hey paulages... you should see the 750 domes i'm putting in my 650... they make the 650 pistons almost look flat.

better check that compression ratio! what pistons are they?

[paulages]

okay, i have some good news for you and some bad...

the 650 head gasket will work just fine with the 550 head. also, the pin height on the two pistons are identical:
(excuse the cell phone pictures)


now for the bad news. there's no way those 650 pistons will work with the 550 head. the head side of the combustion chamber on the 650 is HUGE compared to the 550. the 650 piston won't even remotely fit in the 550 head. you can't really see in the pictures without some frame of reference, but here they are 550 first:

[Soos]

.... i just sold someone a set hoping they would work for him to bore out his 550....
Hope he's not ticked at this info....
would a 650 head drop onto a 550?
I know the 550 cam drops into the 650 head perfectly, and the cam gears are interchangeable. (I was looking into using a "hot"cb550 cam in my 650 so I bought a used 550 cam to test the fit)


And the 750 pistons i'm going to use are 65mm, I initially thought they were 12:1 pistons...
But i'm not too sure...they are a used 836 ART set of pistons from a 750 I bought earlier this year.
The only markings on them are ART and 11 with a ^ over the 11.

I have mocked them up so far on a spare motor using the resleeved and bored out cb650 block.
Not too sure on the valve clearances yet, but the pistons do not hit the 650 head.
Don't want to go too far in the mock up until I have all the parts to re-assemble the '79 motor waiting for these pistons.


It becomes confusing explaining this... unless you know I have 4 cb650 motors.
2 1979 motors -one a daily driver, one completely torn down.
one runnin' '81 cb650c(in a frame)
and one '81 cb650 standard motor being stripped for parts, using it as a tester for deck height for these pistons, as well as contemplating using it as a fixture for frame mount point locations on a 750 DOHC frame.

Did I mention I like cb650's?

l8r

[bryanj]

Hi folks, I have just purchased (awaiting delivery) a Honda dealer workshop manual for the 650 and set up manual for the 650sc so if anyone wants copies i will scan to cd and post for mail cost+a couple of beers.

[paulages]

yes, i'm pretty sure the 650 head will go straight on to the 550 cylinders. that is, i know the bolt pattern lines up. but if you're going that far, you might as well use 650 cylinders, which will also bolt right up... with the cam tensioner modification.  i imagine you could use 550 cylinders, crank, and rods with 650 pistons and head with little problem. the 650 was stroked from the 550, so there are plenty of compatibility problems.

[FunJimmy]

Paul

Do you happen to know what the CB650 bore is. I have a 605 and a 555 big bore kit for my 550 but need a head gasket.

Thanks in advance.

Jimmy

[paulages]

i don't know the actual diameter of the fire rings. the stock 650 piston diameter is 59mm. what size are the pistons on your kits? if you don't use a stock gasket, you'll just need to tell cometic the final bore size and they'll know how big to make the holes.

[Soos]

... the piston diameter of cb650 pistons is 59.8mm.
Not 59.

.8mm isn't much, but it might be a tad over your 605 kit.
What diameter is the 605 kit?


l8r

[FunJimmy]

Thanks for the help guys.

The big bore piston sizes are:

555 Big Bore kit = 59mm
605 Big Bore kit = 61mm

Both of these kits use a one piece oil control ring. I believe the three piece oil control rings are better than the single units.

I also read in an old article “Gentlemen’s Express” that CB750 pistons with modified crowns where the way to go. Maybe the CB650 (59.8mm) pistons with stock rings and a modified crown would be a better way to go. It shouldn’t be difficult to turn the dome to match the CB550 head and compression could be tailored to your own specifications.

Comments?

[Soos]

Thanks for the help guys.

The big bore piston sizes are:

555 Big Bore kit = 59mm
605 Big Bore kit = 61mm

Both of these kits use a one piece oil control ring. I believe the three piece oil control rings are better than the single units.

I also read in an old article “Gentlemen’s Express” that CB750 pistons with modified crowns where the way to go. Maybe the CB650 (59.8mm) pistons with stock rings and a modified crown would be a better way to go. It shouldn’t be difficult to turn the dome to match the CB550 head and compression could be tailored to your own specifications.

Comments?

I would bet the .2 mm difference between the 605 kit and the stock cb650 head gasket would not be a problem at all.

And as for machining the cb650 pistons....

I don't doubt you could machine some off the crown without problems, but as to how much I dunno.
But with a little machining and mabey double up on the base gasket you might be able to keep the compression from going sky high.

If I had a set of cb550 pistons to compare i could make a more informed reply... but I have no 550 parts at the moment.


Anyone have a set of 550 pistons (or even a single piston) they are willing to spare?
I'd like to see how different they are.

Anyone know what the minimum thickness of the dome that is run-able and not blow through the piston?

l8r

[paulages]

Thanks for the help guys.

The big bore piston sizes are:

555 Big Bore kit = 59mm
605 Big Bore kit = 61mm

Both of these kits use a one piece oil control ring. I believe the three piece oil control rings are better than the single units.

I also read in an old article “Gentlemen’s Express” that CB750 pistons with modified crowns where the way to go. Maybe the CB650 (59.8mm) pistons with stock rings and a modified crown would be a better way to go. It shouldn’t be difficult to turn the dome to match the CB550 head and compression could be tailored to your own specifications.

Comments?

I would bet the .2 mm difference between the 605 kit and the stock cb650 head gasket would not be a problem at all.

And as for machining the cb650 pistons....

I don't doubt you could machine some off the crown without problems, but as to how much I dunno.
But with a little machining and mabey double up on the base gasket you might be able to keep the compression from going sky high.

If I had a set of cb550 pistons to compare i could make a more informed reply... but I have no 550 parts at the moment.


Anyone have a set of 550 pistons (or even a single piston) they are willing to spare?
I'd like to see how different they are.

Anyone know what the minimum thickness of the dome that is run-able and not blow through the piston?

l8r

i have a set of 550 pistons you can have for shipping, but i'm telling you, the difference is HUGE. i really don't see how they could possibly work without opening up the combustion chamber quite a bit.