Valve to piston clearance
Im completely rebuilding a small journal 327 (only components re-used are the steel crank and the rods). I have .030 over 10.35:1 pistons and am in the process of selecting a cam, I have my eye on a .501 lift with 1.5:1 rocker arms (unless clearance permits 1.6:1 arms) and I'm concerned about valve to piston clearance. Is there a way to calculate clearance prior to purchasing my cam?
You don't calculate piston to valve clearance, the only way to know is to check it.
valve to piston clearance
You list the compression of the piston. How about a part number. Sounds like a flat top piston with that compression ratio. You need to know the chamber size of your heads to determine the CR. To check the clearance the motor would have to be assembled. You would use modeling clay on top of a piston, bolt the head to the block. You could set up the valve train on the one cylinder to check the clearance. Also you could discuss this with your machinist.
I've never ordered a cam based on clearance, but I suppose you could do it if you wanted to.
Install the crank on oiled bearings and snug the caps. Install a rod/piston assembly (less rings) on #1, #7, #2 and #8 on oiled bearings and snug the caps. Don't torque anything to spec, just hand tighten to snug. Install a degree wheel and pointer on the front of the crank. Find TDC using a positive stop (strap, bolt and nut) above the piston
and mark the inertia ring on the damper if it does not co-incide with the factory notch. The ring on the damper may have rotated a little from its initial register on the hub. If you'd like to begin fresh with a registered damper, get a damper unit from these guys....it has been disassembled, clocked to TDC and reassembled with new elastomeric material. The nice thing about these rebuilt dampers is that you know they will fit your crank snout because they originally came on a running motor from the factory. I have read of considerable belly-aching from fellows who purchase new offshore units and they don't fit the crank, being either underbored or overbored.
Get a set of 8 light checking springs and set up an intake and exhaust valve in #1, #7, #2 and #8 chambers.
Slip your forefinger and middle finger under the retainer and give the valve tips a little tap with a plastic hammer to seat the locks.
Lay the heads on the block and retain them with a couple of bolts, just snugged down. You won't need a head gasket.
Set up a dial indicator
on a flat area of the retainer and preload it by 0.100". Zero the dial. Stoop down and eyeball the plunger on the dial indicator and make certain that it's parallel with the valve stem in both X and Y axes. Caution: push only on the valve tip with your finger or tool. Pushing on the retainer may unseat the locks.
Clearance will probably be tightest on the exhaust valve during the overlap period, when the piston is chasing the exhaust valve back onto its seat, so check it every 5 degrees or so, beginning at 20 degrees BTDC all the way through 20 degrees ATDC on the degree wheel.
You will undoubtedly find varying clearances from one cylinder to another. This will be due to grinding inaccuracies on the crankshaft rod journals, the rod lengths, the piston compression height differences, piston deck height differences, relationship from the block deck to the valve seats measurements, etc. This is why you have to measure every build that you put together.
Now, let's say that you find an average of 0.680" valve travel before you touch the piston with the valve head, as you push the valve tip down. If you add the gasket thickness (let's use 0.040"), you have the capability of using a cam with (0.680" plus 0.040" = 0.720") at zero clearance. If you use the generally accepted figures of 0.080" clearance on the intake valve and 0.100" clearance on the exhaust valve, then you should be able to (in an otherwise perfect world) run a cam with up to 0.620" lift without interference.
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