Topic: Valve to Piston Clearance

[bunghole]

I've installed the Wiseco 836 pistons into my '77 750K.  I'll be using new stainless valves and a cam from Dynoman.  I imagine I will need to measure the valve to piston clearance with the new setup.  What is the best and/or easiest way to do that?

I was thinking one way would be to measure the "worst case" clearance by marking the valve stems with a sharpie at the highest lobe lift and then installing the head over the pistons at TDC and measuring clearance at that point.  Anyone see any flaws in that idea?

[City Boy]

I would suggest placing a layer of plasticine on top of the piston .Assemble and turn engine through a few complete rotations.Remove head,slice through plasticine and measure thickness of untouched material.that is your valve/piston clearance.   Rock On

[martino1972]

I would suggest placing a layer of plasticine on top of the piston .Assemble and turn engine through a few complete rotations.Remove head,slice through plasticine and measure thickness of untouched material.that is your valve/piston clearance.   Rock On

+1 on that,thats the only proper way of doing it...

[bunghole]

Thanks guys!

Plasticene is just a fancy way of saying modeling clay right?

[martino1972]

Thanks guys!

Plasticene is just a fancy way of saying modeling clay right?

    yep,spray a little wd-40 or something on the valves,so that they dont stick to the clay,otherwise it will pull the clay up and you cant get a reading...
also make sure you dont use 2 pounds of it on each piston,will be a hell of a job getting it away from the rings etc..
oh,and just to be sure,the head will have to be torqued down for a correct reading...

[Pinhead]

So basically, the engine has to be completely put together with the clay on the pistons, and then completely dismantled to take the reading... Too bad there's not an easier way.

[fastbroshi]

I always wondered if this process would render the head gasket useless.  Should it be chalked up to breakin a few eggs?

[bunghole]

I'll probably lightly torque it without a head gasket.  That way there will be a tiny extra bit of margin and I won't have to kill a gasket.

[martino1972]

depending on the thickness of the headgasket,you might end up touching the pistons with the valves.if you dont have a headgasket in there....
it shouldnt hurt the headgasket if you do this measuring with the headgasket in there....

[bwaller]

Clay is good but is sometimes a #$%* to get a good accurate read. On assembly when you degree the cam and with a degree wheel in place, mount a dial indicator with the pin on the valve spring retainers.  (both in & ex) Check a few degrees before and after TDC when the pistons are closest to the valves. Gingerly pry the valve open until it just kisses the piston and take the readings on the dial indicator. 0.080" exhaust and 0.050" intake is safe.

[Triffecpa]

if you are going with an "off the shelf" piston and cam combo, it's unlikely that you'll run into piston/valve clearance problems.  However, if you're decking the head to increase compression even more, then you'll need to double check clearances.

Tracy

[Pinhead]

Wouldn't it be possible to check the clearance before pulling the head? In other words, pull the cam rockers and valve springs, move the piston in question to TDC, move the lobe to its highest point, and measure the clearance between the rocker and the valve stem?

[bunghole]

Wouldn't it be possible to check the clearance before pulling the head? In other words, pull the cam rockers and valve springs, move the piston in question to TDC, move the lobe to its highest point, and measure the clearance between the rocker and the valve stem?

That's sort of what I was thinking as this should theoretically be the closest the two could get to each other, no?

[bwaller]

NO. The closest potential contact point is rarely at TDC. The intake usually is closest after TDC. That's why it's a good idea to check at different points either side of the top.

Remember not to force rotation if you feel any resistance when checking.

[hymodyne]

I'm going to try this, with a 650 cam in my 550 head. I've already dremeled out the areas where contact was  most likely between the lengthened lobes and the  head. I read through the post, "650 cam into 550" and found nothing discussing the potential for valve/cylinder contact.

I thought of dripping wax over the valve reliefs in the piston tops before fitting the whole thing together--using an old head gasket.

hym

[754]

Time has shown that it is easier, to take the time & check..
than to deal with the aftermath if it was not right..

 Its just good engine assembly practice, do it , or you may wish you had.

[MRieck]

Clay is good but is sometimes a #$%* to get a good accurate read. On assembly when you degree the cam and with a degree wheel in place, mount a dial indicator with the pin on the valve spring retainers.  (both in & ex) Check a few degrees before and after TDC when the pistons are closest to the valves. Gingerly pry the valve open until it just kisses the piston and take the readings on the dial indicator. 0.080" exhaust and 0.050" intake is safe.

That is the method I use. You can get by with .060 on the exhaust no problem. I machined an old tappet cover make the prying easy (you install the cam cover with 4 or so bolts), install the modified cap and pry down. I'll take a pic of it later today.

[MRieck]

Time has shown that it is easier, to take the time & check..
than to deal with the aftermath if it was not right..

 Its just good engine assembly practice, do it , or you may wish you had.

I agree with too.

[754]

There is a quick method that can be used.

I think with engine assembled, less valvecover. You simply put a screwdriver or alum prybar on the rocker/cambearing (other side of the rocker tip & under the rocker), and pry up at full lift. With a dial indicator on the valve collar you will see exactly how much you have.