Topic: Acceptable clearances between the piston/head and the valves ?? any #'s??

[Soos]

I've got a set of Pinheads cb750 on the way to me, and I am going to be doing some machining to them for valve pockets(at a minimum I think, probably deck height and such will need worked on)
I also have a set of cb750 automatic pistons I am eager to compare to his '77/'78 high domed cb750 pistons. Plan on giving them the same treatment as Pinheads.


Pinhead is asking me to get things as tight as possible(to bump up the CR) yet maintaining proper clearances between everything. I would hate to send him some engine grenades.
 


On Pinheads pistons I am going to do a fair bit of verifying clearances in order to help up his CR.
I'm guessing 2-3 times claying, measuring, and re-machining the pistons as needed to get to the minimum clearances I can get it to run with.
Once I have 1 piston "adjusted" I can copy depth, diameter, location of valve pockets/deck height/dome height/contour to the other 7 pistons in one run.
I have done this to my '79 650 and have the setup parts for verification, and access to the machinery to do the machining needed.


Just a little lost on the clearances issue......





In this post:
http://forums.sohc4.net/index.php?topic=39029.0
MReick posts keeping 0.060 valve to piston clearance.


Anyone on the recommended clearances for the intake?
About the same I would imagine, what is the min. allowable clearances?


Pinhead is mainly gunning for getting every ounce of power used efficiently to get a really good MPG bike I believe. High CR helps from what I understand.
So he will probably not be in the 10,000 RPM range more than a 2-3 times, i'm thinking he will be more in the 2-8k range for RPM 99% of the time.
(possibly more like the 2-7k range 95% of the time i'm guessing)

Due to his lower operating RPM's I am tempted to give in the range of 0.050 clearances for the valves.
I'm thinking there will be little if any valve flutter under 8k rpm.



Oh BTW, his motor has a few threads on it already on the "surface turbulence" cb650 motor work he has already done.


I'll add the links them as I find them.
But basically these pistons('77/'78 high domed cb750 pistons) are going into a '79 cb650 motor, stock cam, head, and valves.

[754]

Did you check clearance on a stock piston?

[turboguzzi]

1.6mm / 060" for piston to valve is very safe,

you can go as low as 1mm / 040" piston to head.  with the deep CC of a 750 i really doubt you'll have a clearance problem in the first place

[Bill/BentON Racing]

+1 on what Mike and TG said.Bill,Less than .040 not recommended!

[Soos]

Did you check clearance on a stock piston?

No but I think I might now.


l8r

[turboguzzi]

soos

had a quick look at that 650 turbulence motor, how did it work out at the end?

[MRieck]

1.6mm / 060" for piston to valve is very safe,

you can go as low as 1mm / 040" piston to head.  with the deep CC of a 750 i really doubt you'll have a clearance problem in the first place

I'd stick to .040 on the intake side only

[Soos]

 


They don't make a big enough smiley emoticon for that response there MReik!




And turboguzzi, thats Pinheads motor, not mine. And no it's not quite done yet I think.
He just recently got the pistons in the mail.... let alone machined and back in his motor!


I had thought of doing the surface turbulance bit once(after reading pinheads posts on it)... and thats as far as that got for me.
I thought a head and bore job was enough.(and I smile every mile BTW)




One Q. for you Mreik :

HOW reliable is a motor with those clearances(0.060 exhaust, and 0.040 intake) when compared to a stock motor?
Is it (as I am assuming) negligible in the sane RPM range(sub 8k rpm)?



thnx all



l8r

[MRieck]

They don't make a big enough smiley emoticon for that response there MReik!




And turboguzzi, thats Pinheads motor, not mine. And no it's not quite done yet I think.
He just recently got the pistons in the mail.... let alone machined and back in his motor!


I had thought of doing the surface turbulance bit once(after reading pinheads posts on it)... and thats as far as that got for me.
I thought a head and bore job was enough.(and I smile every mile BTW)




One Q. for you Mreik :

HOW reliable is a motor with those clearances(0.060 exhaust, and 0.040 intake) when compared to a stock motor?
Is it (as I am assuming) negligible in the sane RPM range(sub 8k rpm)?



thnx all



l8r

It reliable enough as long as the springs are appropriate for the cam and you don't bounce the valves . I have run intakes down to .030 on 600 Supersport race stuff...that with good springs and stock cams. Those valves are pretty light....keep that in mind

[Soos]

Good to know on the racing side....
I might be tempted to get (really)near that area for my next personal motor overbore if I go with better springs/keepers/retainers.
I am seriously tempted to run at the flats one of these days, and every ounce of extra power helps.



l8r