[snbush67]

Originally Posted by MoreGAS

You guys need to be careful here as TDC is not always the point where the piston is closest to the valve on either intake or exhaust sides. If you use a degree wheel to set cam timing you can get a very good picture of the nature of your camshaft(ramps on lifting and closing sides and rate of increase in lift per degree of crank rotation) , and when the valves are the closest to the piston and often it is roughly 10 degrees away from TDC I recall(note book not near me) on the intake side. This is especially true with more aggressive cams as the intake valve opens 20-30-40(or more) degrees BEFORE the piston is up at TDC.

Here is how we check this: Set your cam to desired spec, after you have hopefully found true TDC w a piston stop and a degree wheel as the pulley marks and fan housing's are often not right on, then then run the intake valve in 1mm at at time initially from 0 lash (1 turn is .040" and you can watch this with your micrometer still on) . Turn the engine around 2 revolutions gently as you get towards TDC on firing no . 1 . Back out the exhaust lash to a whole bunch 1st I should note so valves don't tangle. Then go again in 1/4 turn increments or .010" . Keep going until it hits, and you will see it is very very often not at TDC. The intake valve is opening long before TDC getting a head start on the intake stroke at the end of the exhaust stroke. Once done back it off about .005 and you learn your exact clearance within .005" or so. Pro tip- Do this with only one cam timed or you will have to redo both sides if you have to readjust your cam timing because you have inadequate clearance on one valve or the other.

We record this info as if we have a problem later, we learn exactly what we can get away w certain cam, valve train spec, springs, retainer, valve weights , spring pressures etc., level of over Rev (from data loggers). We of course have limits we know not to exceed on how close we can get away with, and some clients are much harder on equipment than others, so all this needs to be accounted for, and over revs can exceed what one dreamed was even possible.

Then we do the exact same process on the exhaust side, cam still at same timing spec, but now you back off the intake valve rocker several turns from TDC firing NO 1(both valves closed) , and run in the exhaust rocker .040", .060, .080" .100" ...till it hits. Turn the engine slowly 2 turns (all spark plugs out so easier to turn) once you get in to the area where it hits around but not at TDC likely. Then you know almost exactly what you have for clearance. Recall the piston is chasing the Exh valve shut on the exhaust stroke( and this is why a big over rev often breaks a rocker = tags the exhaust valve to piston often -valve float or loss of control- and hopefully doesn't bend the valve ), and the valve doesn't actually close until longer after TDC, 30 degrees on a more racey "cammed" engine.

We use this data to then modify our custom piston designs so we don't have one size fits all" giant ass valve pockets" we don't need for next engine like this we do to the same spec(all things equal) . Piston manufacturers commonly make these pockets very deep to avoid come backs from folks having clearance issues, especially off the shelf designs. Streetcar designed pistons have very little valve reliefs so then one cannot fit much additional camshaft, a real bummer. Sometimes there is not enough dome thickness to cut your own valve reliefs, so a different piston or milder cam is required to go forward. This is often why from a new or totally different design or spec standpoint, we end up building an engine 2X, once to chk everything fits", and then the final time after some mod's perhaps. We have had Mahle give us a 3D printed custom piston model to mock up, to make sure the design fits before producing real parts which can waste time and material if a detail missed a bit.

Happy engine-ing

BeST

Kevin
GAS Motorsport

Thanks Kevin,

Great info, yes these pistons have deep pockets, I’m probably going to end up building it again later with twin plug, no base gaskets, higher compression and closer clearances. I did use a piston stop and am using a degree wheel so I have that going for me.

My notes show my exhaust valve closes at 15 degrees, I’m going to go back and check.