I am curious, to those with WAY more experience than myself do you think that the combo of highdome 302 pistons in a 240 with a heavily modded 300 head will kill itself. I am shooting for a high compression 240. However the reason that I am concerned is that I do have oversize valves and am planning on running bigger ratio rockers with a race profile cam. I would hate not to have double checked my math with you guys first put the thing together and watch my ignorance come life with a loud bang, 12 bent valves, and a puff of smoke for good measure.
Piston relation to valve clearing
- Thread starter MoFoPo
- Start date
8) this is why you have to check the valve to piston clearance when assembling an engine. mock up one piston/rod combo, install the cam with a solid lifter, and install the head after putting some modeling clay on the piston dome. then using a ratchet, turn the crank through two revolutions. dont force it if you hit something solid and cant turn the crank, but recognize also that unless you have installed a pair of light valve springs, you will have some points where it is hard to turn the crank. if you get two clean revolutions of the crank, carefully pull the head off, and remove the piston. now carefully cut the modeling clay at its thinnest point, a razor blade works real nice here, and measure the thickness of the clay. if you have .080" on the intake side, and .100" on the exhaust side then you are good to go. if you have less than these number, then take all your pistons to a machine shop and tell them you have x amount of clearance and you need .100 on both sides(that is easier than telling them two different numbers and hoping they get it right since they may not get to your job for a couple of days.)
Howdy MoFoPo:
Math and measurements are great, but are no substitute for a trial fit prior to the final button up. When using non-standard or altered by machining parts it is a good idea to assemble all with a piece of modeling clay on the top of the pistons. Next, roll the crank assembly over by hand, slowly. IF anything binds or interfers stop and assess. If not, roll it over through several revolutions, then dissassemble. Inspect the clay at the tightest, narrowest point and carefully measure for valve to piston clearance.
A fairly safe generality is to maintain .100" piston-to-valve clearance on the intake side and .120" on the exhaust.
Hope that's whatt you needed.
Adios, David
Math and measurements are great, but are no substitute for a trial fit prior to the final button up. When using non-standard or altered by machining parts it is a good idea to assemble all with a piece of modeling clay on the top of the pistons. Next, roll the crank assembly over by hand, slowly. IF anything binds or interfers stop and assess. If not, roll it over through several revolutions, then dissassemble. Inspect the clay at the tightest, narrowest point and carefully measure for valve to piston clearance.
A fairly safe generality is to maintain .100" piston-to-valve clearance on the intake side and .120" on the exhaust.
Hope that's whatt you needed.
Adios, David
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rbohm":39mpph9f said:
THIS IS HOW I CHECK IT ALSO AND MAKE SURE YOU DO IT WITH OUT THE HEAD GASKET!!!! THEN U HAVE THE THICKNESS OF THE GASKET AS A EXTRA SAFTY MEASURE
Keep in mind that the max valve opening doesn't always correspond to TDC. The relationship between the piston and valve is dynamic, so the valve may be either opening as the piston is pulling away or closing as the piston approaches. Your cam timing will greatly affect clearance.
rbohm's clay technique is a good procedure to follow and well worth the time.
rbohm's clay technique is a good procedure to follow and well worth the time.
if you use low tension(2-3lbs) checking valve springs (such as Crane #99881-2) you can use a dial indacator on the spring retainer to measure directly and you will get the reading to .001 and not have to mess with the clay. However I'd do both as the dial inacator will not tell you WHERE the vlavle is the closest to the piston.
A dial indicator will tell you how much the valve is opening, but it won't tell you piston-to-valve clearance. That's why you need the clay.
MustangSix":1itdwua2 said:
A dial indacator will tell you valve to piston clearance. You just have to set in a different way. when you have the valvle lifted off the seat and want to measure clearence you zero the idacator on the retainer and push the rocker down. You then get the number. now you need to repete this both btdc and atdc to find the closest point. to do this you almost have to use the low tension springs.
The clay has its place and that is telling you where your clearence is the closest. But just how do you measure clay and telll the difference between.
.080 and .090?
The point of nearest contact is not always at TDC. Like I mentioned, the valve to piston relationship is always changing as the engine rotates, and depending on cam specs, a valve may be opening as the piston is moving away from it and closing as the piston approaches. Measuring by pushing the valve until it contacts the piston won't tell you what you need to know because the piston may not be in that location when max valve lift is achieved. Even with the same cam you could alter PTV clearance by advancing or retarding the cam and changing that relationship.
You measure the clay by slicing thru it and measuring the cross section with a caliper. You wont get a measurement to the last .0001" for sure, but you're only looking for any value greater than .080" as a rule of thumb.
You measure the clay by slicing thru it and measuring the cross section with a caliper. You wont get a measurement to the last .0001" for sure, but you're only looking for any value greater than .080" as a rule of thumb.
MustangSix":1qra90nv said:
Sorry I can't belive I left out the most imortant part. I ment to say check it at TDC AND every 10 degrees in both direction so you can zero in on when you get the closest.
It really gets wild doing this on harleys and hemi's as you also can get into valve to valve issues.
80broncoman":34z6uvpi said:
that's a good method with OHC engines or other ones that have complex head assemblies, but on our little pushrod sixes I'd say it's probably easier to just use the clay.
