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Delete the base gasket?
- Thread starter Punkskalar
- Start date
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- Tags
- engine
motor in question is a "256" w/o the cam chain blocking bridge which makes it a 30something... i can never remember.
glenn- thanks for the immediate and very detailed response. i did not adequately account for the slight change that total stack height has on relative cam position, which in this case is governed entirely by the cam chain. i edited my post a few minutes after yours- does the "you're good to go" still apply? unless i'm badly mistaken, there is no way to mod this particular cam sprocket. first time i've gotten this far into this process, forgive me.
2Many- so if we're only talking .3% bump in compression, no sense in re-inventing the wheel i guess...
glenn- thanks for the immediate and very detailed response. i did not adequately account for the slight change that total stack height has on relative cam position, which in this case is governed entirely by the cam chain. i edited my post a few minutes after yours- does the "you're good to go" still apply? unless i'm badly mistaken, there is no way to mod this particular cam sprocket. first time i've gotten this far into this process, forgive me.
2Many- so if we're only talking .3% bump in compression, no sense in re-inventing the wheel i guess...
Something else to consider. That 0.4mm height reduction translates to about 1/2° of cam rotation, about 1° at the crank...
How much do you owe Paul?
Yamaha also changed the pistons during the XS1B production run so the compression was lowered to 8.1:1.
Yamaha played with compression ratios throughout the XS run and from 8.7;1 down to 8.1;1, 8.3;1, 8.4;1 and then back to 8.7;1 around 78 and run that compression until end of production.
Early 256, motors had a rep of throwing the small end cooling the engine with a window in the front of the engine. Beefing up early motor compression's, imo, will only make this problem worse. Considering your bike will not be a racer i would be looking at other ways to get more from the engine. Head work and carbs with better response like the EX500 used.
Attachments
really appreciate the input of all above. i was not paying enough attention to the subtle change that playing with base and head gasket thickness has on relative cam position. like most things in life, there is no easy shortcut here...
650Skull- thanks for the reminder re the 22mm wrist pin setup. i'd read that on the forum back when i bought a bunch of these bikes. in the course of tuning VM's for these motors on the street it's surprising how much riding time one spends at 1/8 throttle or so... all of my builds are stripped down and race-ish however; i do put them through their paces on the backroads but always mindful not to redline them much if ever.
ya guys scared me off. will install the base gasket during re-assembly and get back to finishing the porting.
650Skull- thanks for the reminder re the 22mm wrist pin setup. i'd read that on the forum back when i bought a bunch of these bikes. in the course of tuning VM's for these motors on the street it's surprising how much riding time one spends at 1/8 throttle or so... all of my builds are stripped down and race-ish however; i do put them through their paces on the backroads but always mindful not to redline them much if ever.
ya guys scared me off. will install the base gasket during re-assembly and get back to finishing the porting.
glennpm
Another Old Biker Nut!
"ya guys scared me off"
Hey don't be a scaredy cat ;-)
Hey don't be a scaredy cat ;-)
Checking the cam timing would be prudent anyways.
Here's some more threads, covering reindexing the cam sprocket:
http://www.xs650.com/forum/showthread.php?t=6867
http://www.xs650.com/forum/showthread.php?t=12847
http://www.xs650.com/forum/showthread.php?t=29831
http://www.xs650.com/forum/showthread.php?t=34965
http://www.xs650.com/forum/showthread.php?t=38556
Here's some more threads, covering reindexing the cam sprocket:
http://www.xs650.com/forum/showthread.php?t=6867
http://www.xs650.com/forum/showthread.php?t=12847
http://www.xs650.com/forum/showthread.php?t=29831
http://www.xs650.com/forum/showthread.php?t=34965
http://www.xs650.com/forum/showthread.php?t=38556
guys, my earlier question got missed... will rephrase (not rephase 
) it:
does EVERY year XS cam have a press off sprocket?
at a glance i assumed that the early ones were one piece, hence my confusion. but i think i'm wrong based on comments . pardon my ignorance.
i see there's an Ebay seller offering pre-indexed coated cams with .016" copper head gasket, but not for the early motors which don't support as many mods...
) it: does EVERY year XS cam have a press off sprocket?
at a glance i assumed that the early ones were one piece, hence my confusion. but i think i'm wrong based on comments . pardon my ignorance.
i see there's an Ebay seller offering pre-indexed coated cams with .016" copper head gasket, but not for the early motors which don't support as many mods...
As far as I know, all 650 cam sprockets are pressed on. In fact, swapping a later 18T sprocket onto an early 256 cam is how you go about adapting it for use in the later 447 motor. The 256 cam is supposed to be a bit "hotter", hence the mod. Here's a thread detailing the procedure, note pressing the sprocket on is not recommended, just pressing the sprocket off. It's safer to install using heat .....
http://www.xs650.com/forum/showthread.php?t=29831
http://www.xs650.com/forum/showthread.php?t=29831
Yes, they are all press fit. I don't have a lot of confidence in my ability to control a torch well enough to avoid overheating the sprocket, so the last time I did the job I heated the sprocket to 450* in an oven and chilled the cam in a freezer. Those temps didn't let me just drop the sprocket on, but it gave me enough to start it, line up the marks, and finish driving it home in the press. In using the press you need to be very careful that you've started the sprocket without cocking and support the work so that it's level. I found a camshaft in a salvage XS650 motor that showed what happens when those things aren't carefully done; it had a big chunk broken out of the sprocket boss.
And before somebody asks: Yes, 34t and 36t camshaft sprockets have the same ID.
And before somebody asks: Yes, 34t and 36t camshaft sprockets have the same ID.
ok, that clears that up, and very helpful tips re sprocket removal/install. now i know more about these cams than i did a couple days ago thanks to all above.
the scales are tipping toward "use the base gasket". the good news is with the '73 motors you have the hotter cam and longer stroke, bad news is you're not supposed to push them as hard. i should probably just pick up a later motor down the road to get more creative with.
the scales are tipping toward "use the base gasket". the good news is with the '73 motors you have the hotter cam and longer stroke, bad news is you're not supposed to push them as hard. i should probably just pick up a later motor down the road to get more creative with.
after all the base gasket "to be, or not to be" stuff, this is really quite simple.
the new aftermarket head gasket is .060". OEM head gasket as removed is .045". so with most of these modern replacement gaskets you should omit the base gasket anyway. .003" or so net stack height difference is negligible.
the new aftermarket head gasket is .060". OEM head gasket as removed is .045". so with most of these modern replacement gaskets you should omit the base gasket anyway. .003" or so net stack height difference is negligible.
Why would you delete the base gasket and conserve deck height with a thick head gasket for no change in compression? BTW, Vesrah produces head gaskets to OE thickness and quality, and Ottoco head gaskets are .040", a little thinner than OE. AFAIK Athena is the only maker that produces 1.5 mm. head gaskets; maybe some Brand X junk out there is made to that thickness too, but it's not typical of aftermarket head gaskets as a whole.
ok, good to know... i did not mean to suggest that every gasket set out there has an .060" head gasket, but that's what's in this set. they appear to be well made. the last set i used may have been Vesrah.
but yes, to whom it may concern, measure your gaskets before you install them.
but yes, to whom it may concern, measure your gaskets before you install them.
