Valve to piston measurement

[kent olsen]

Mike Braun at J&B racing has been helping me with my rebuild but they are closed for the weekend so maybe the board can help me.

J&E pistons 9.5:1, GT2-102 cam with intake lift at the cam of .485 and exhaust of .470.

Task: measure the distance the valves travel before they touch the piston. Set piston #1 to TDC. Install the heads for #1 & 3 and just tighten snugly. I DID NOT install the head gasket. Install the cam tower and again tighten snugly. Install the cam. Back the crank to, I believe it's 7 deg BTC, marked on the pulley by Mike. Mike had already replaced the valve springs with light weight springs. Install rockers. Turn the cam until just touching the rocker. Tighten the rock to zero lash. Tighten the rocker, extending the valve, until it touches the piston, counting the turns (1mm per revolution). Do the same thing with the exhaust side with the piston set to Mikes mark, I believe somewhere near 7 degs ATDC.

My question is this. I got 9.7mm (.380") for the intake and 8.6mm(.336") for the exhaust. Even if I add the thickness of the head gasket (.11") I'm still going to hit the piston.

Either I did something wrong, the head gasket, or the lift at the cam (John Dougherty's data) is not the same as the distance the valve moves into the cylinder.

[camgrinder]

Hello Kent,

Your GT2-102 cams should install at 4.0mm lift at tdc on split overlap. At this setting the intake valve at tdc will be open 4.00 mm and the exhaust valve will be open 2.95 mm.
To properly check the V-P, I would set the camshaft timing to 4.0 mm. Set valve the valve lash. Bring the engine to TDC. Double check the valve is open 4mm and push it down until it touches the piston. This will be your clearance at tdc. From the sounds of it, you have 5mm plus clearance. You can also check it 15 degrees before and after TDC to be sure, if your close at TDC. Remember, full valve lift on your cams is 102 degrees after top dead center on the intake, and 102 before on the exhaust. The piston is more than halfway down the bore.
If this is confusing, please call me. John 530-672-2286

[RichHawk]

subscribed. Because I'll be doing the same check in the next week with my DC20 cams.

I understand everything John says except for how you lock the cam at 4mm lift (.157" opening). Obviously to measure the 4mm lift is the dial indicator and Zbar setup. But wont the spring pressure try and roll the cam back off?

What do you use to secure the cam? no scratch that, John probably has a custom made proper tool for this.
What would I use to hold the cam still as a one time tool crafted from garage junk?
Richard

[Eagledriver]

There are several issues in here that are wrong or need explaination. The closest approach of the valves to pistons are NOT at TDC. The intake valve makes it's closest approach just after TDC because the valve is opening rapidly at TDC and the piston has not started down the bore much for the first 10 degrees or so. The same situation occurs for the exhaust but the close approach is before TDC. In the very complicated and incomprehensible (to me) post at the top it sounds like he is checking the intake before TDC and the exhaust after. This is backward. John is correct about how to check clearance at TDC but that doesn't tell you how much piston to valve clearance you have at the close approach described in this paragraph.

I check the clearance at several points around TDC plus or minus 10 degrees to get a good feel for how close the valves get. You only need about 1mm on the intake and 1.5mm on the exhaust (if you are finding the actual closest point).

-Andy

[colescrogham]

eagle has good advice. You noted that there are "soft" springs already installed, so as you move the crank position in several different positions close to TDC, it is pretty easy to set up your dial indicator and give the rocker arm a little push. The spring will allow the valve to touch the piston and you can see exactly how close the valve is to the piston at that crank position. It won't take you long to figure out where the closest point will be where the piston and valves (remember, they move at different "rates" of speed) have minimum clearance.

[kent olsen]

John
Remember those cams cores that got lost? Well they must have been the older ones because your cams needed the bolt in the end. No problem but something else I didn't have.

Well it sure ain't no small block chevy!!!!!!

Since I'm trying to get this thing together this month and I'm going back to work with little time off the rest of the month I bowed to the experts. I loaded her up in the truck and took the engine to J&B racing. Mike Braun, Jim and Beverly have been a tremendous help. I can't say enough about how much they've helped me. I highly recommend them to anyone.

Anyway while I'm gone to work Mike is finishing up the top of the engine. All I'll have to do when I get home is install the rockers. Next will be pulling the 2.7 from the car and swapping over the oil cooler, intake and exhaust, and the hyd chain tensioners.

Thanks everyone for your advise.

[Jeff Higgins]

Kent, you should have plenty of clearance. As you know, I have the same cams in my 3.0, but with the JE 10.5:1 pistons. Mine had plenty of clearance. I followed the clearance check procedure as outlined in Wayne's book, checking it every few degrees of crankshaft rotation. I don't have my spread sheet in front of me, but I don't remember anything even close to what one would consider even minimum clearance.

Like John says, the valves are not all the way open when the piston is at TDC. The tightest clearances are somewhere below TDC and at somewhat less than full lift. The valves "chase" the piston down the bore as they open and the piston descends, then the pistons "chase" the valves as they come back up and the valves are closing. It's hard to predict where they will be the closest to making contact, so we need to turn the crank and measure at several places in its rotation. Either that or use the old clay or solder crushing trick, but that won't tell use where in that rotation they get closest.

[kent olsen]

Jeff

I watched Mike Braun do the measurements and you are all right, there's plenty of room. The compression came out to 9.4:1 when Mike measured it. That's with J&E 9.5:1 pistons so it looks like everything is falling into place. I did drill the second spark plug hole so when I can afford the distributor I'll go with the twin plugs, which should be a couple more horsepower.

[fredmeister]

I would suggest checking with clay on the piston tops to see the minimum clearance thruout the range of motion. This will not tell you where it occurs, but the minimum is what you need to gage anyway.
It is just a pain to set up and then tear down the engine again during the test.