Topic: Megacycle 125-05 cam - help

[750deepsouth]

I'm at the stage in fitting this cam to my Cr750 and run into a problem - there is no piston/valve clearance. I was lead to beleive that the 125-05 grind would drop straight into a standard 750 motor with no clearance issues. anyone with megacycle cam erxpereience ?

Thanks,

Andy

[MRieck]

That cam is only about .020" more lift on the intake side...clearance shouldn't be an issue. I'd guess your cam sprocket is off a tooth or 2.

[750deepsouth]

Mike, thats what I thought. I suspect the guy that ground my cam may have ground it to incorrect specs, ie to one of the racier megacycle grinds. However, he says it was done to 125-05 specs so I can't really argue with him at the moment.
My mechanic was thinking it could have been a duration issue - ie the cam lift is only a faction more, but being held open longer is the problem..
Do you know what the standard clearance would be piston/valve ? what sort of leeway in there. There are pockets in standard pistons so I guess its not large..

[MRieck]

Standard piston to valve is probably something like .080 to .090...maybe even a little more. You can safely run .040 intake and .050 exhaust. The pockets on the OEM pistons are more of a groove than a pocket. I would agree that extra duration would be more of problem than overall lift. The exhaust is hitting I presume? Are you using an adjustable cam sprocket? Has the head been milled or cylinder decked?

[750deepsouth]

Ok. Yes, I have a standard piston and its not a very pronounced pocket alright.

Both valves are touching - just. Yes, we got a slot milled in the sprocket so it's adjustable. No, the head is not milled nor cyclinder decked.
All I can think of is that the aftermarket pistons I am using don't have the same pocket the NOS Honda ones do.
{ie I bought a piston kit 0.5mm oversize but they were not NOS Honda, I can't quite recall the maker}

Thanks for all this help.

Andy

[MRieck]

 OK..now it makes more sense(non OEM oistons). I wonder if the wrist pin hole is a little lower than stock which would raise overall height. Looks like you will have to flycut the piston reliefs .080 or so. Check the overall lift on that cam with your dial indicator to make sure it is the 125-05.

[750deepsouth]

I thought about the wrist pin height and considered that unlikely - but we are talking about very small margins here, so it can't be discounted.
Its the usual story, you can only count totally on the genuine Honda parts.
And yes we will check the overall lift - that will confirm the cam grind.

[Sam Green Racing]

Good luck Andy I still have this to come although Mike has asured us that the pistons we have will clear.
Will get the pics to you shortly, have had hell of a job finding a battery for the camera.

Regards Sam.

[750deepsouth]

Thanks Sam. At least you have a camera - I am without at the moment !!

[Sam Green Racing]

It's not a digital camera Andy, thats the reason I cant email the pics to you.

[Sam Green Racing]

Andy,  check your email.

[750deepsouth]

Sam, Ok got that ok. Thanks very much.

Andy

[Sam Green Racing]

How have you got on with the pistons/ cam problem.

[750deepsouth]

I reluctantly decided to go back to a standard cam at the moment. Reason  ~  $$.
I don't have the expertise myself for motor work and costs are getting high. I have two kids at University so I have to be a tight at the moment. Any parent in this situation will know what I mean. 
In 18 months or so I'll be in a better position to tackle the problem.