Piston/valve clearance, radially

[FJman]

Okay, first I apologize. This isn't about a GS. It's about a FJ1200. I'm justifying it by saying the concepts should carry over


I know that I need a given amount of clearance between the piston and the valve, and that it doesn't happen necessarily at max lift or at TDC. Happens where it damn well wants to. That's okay, I seem to be okay when I put clay in the head and spin things around.

Here's where the fun starts though. The build is a stock bore (77mm) wiseco 10.25:1 setup (compare to stock at 9.7:1), but with oversized valves in the head.

The intake valves are the issue here. The reliefs in the pistons are the same size as the valves (30.5mm, but that's unimportant). So when I'm timing the cams in and measuring away happily, I've got lots of room between the piston and the valve if I actuate the valve an extra 0.125" at any point in the cycle.

However... the outer rim of the valve against the indented valve relief area? Not much clearance. Not much at all. 0.015", maybe? Is this enough? I know the intake valve is going to slightly expand when it's bathed in fire every other revolution, the piston is going to grow a bit as it heats up, and I'd be shocked if the valve didn't wobble a bit during operation at (up to) 10krpm.

What's safe for a clearance here? I can get the pistons cut a bit, not deeper, remember, but larger reliefs, but I'm hoping not to. I tried to set the engine up in such a way that the valves stay an absolute minimum of 0.050" from the piston, but it puts the centers so far apart that I get a max of 105 psi in the cylinder, not good for power!

Advice, help, tips, all appreciated. Being told to buy a suzuki, not so much (I'm going to get a 9.xxx pass on the FJ at 58" if it kills me!)

[rapidray]

You will be ok, just make sure you have a rev limiter on the bike. The biggest hurdle for you running in the 9s is your wheelbase. 4-6" more swingarm will make a WORLD of difference!!! Ray.

[trippivot]

were the pistons bought with bigger intake pockets?

what you said is unimportant IS important.

when setting up your engine / head installation/ the piston crown has to be clearanced for the bigger valve. the valve has to fit all the way to the bottom without touching the edge of the piston valve pocket. checking clearances with the head flat on the cylinders and blue ink. What is important is that the valve seats fully in the pocket equally - including the "squish band piston edge". not touching at all except the bottom. setting with the head at 0 is the way to go, as when the head gasket is installed there is space created. This is necessary before assembling a hi- rev engine.

if the pistons were fly cut at a machine shop they might need to go a little bigger.

but the piston pockets have to be a little bigger than the valve itself and it has to fit in there with out restriction. If you have that, then the 2 surfaces will not smack into each other. The closest measurable clearance of interfering / recriprocating parts will be the bottom of the pocket - not the edges.

[FJman]

Good info, tripp! You got the idea, but peripherially. Now I've learned that another way to say it is "eyebrow clearance."

The valves fit in the reliefs of the piston freely, they sit flat at the bottom (holding them in my hands), but the clearance is really tight, like .015" or so. When I measured it, I was using clay, so I wasn't able to get perfect measurements on the mushy stuff, but I was able to see go/nogo clearance. My concern was exactly what you're describing, that the piston would clip the valve right at the edge, rather than on the bottom of the valve. Clearance between the bottom of the valve and the valve is something enormous, a quarter inch or better.. which explains the 90-100psi compression reading that I got. My lobe centers ended up really, really far apart--I didn't measure, I just kept opening things until the piston and valve were never closer than .075 or so at any point, and the tight spot was where you said it'd be, along the edge of the valve (not the bottom) against the side of the valve relief... it wasn't at the bottom of the relief, but that's because the valves never travelled that far down into the pocket with things set so conservatively! Actual piston/valve clearance measured from the bottom of the valve to the piston top was way, way high...

So yes, the question really is most clearly stated as "what's a safe clearance for the valve edge to the edge of the piston's pocket?" The valves I believe are stainless, the motor is air cooled, and will be raced with a nitrous shot on it... I'm concerned that the additional heat from the nitrous will raise cylinder temps, causing the pistons and valves to expand and kiss each other.

The FJ motor specifically has a semi-crappy design on the bottom end, and makes peak power at 8500 from the factory, redline at 9500. If the motor gets spun too hard, there's a poorly supported section of crank that flops about and eventually breaks the crank down the middle, so a rev limiter is going to be installed to shut the spark off at 9750.

Ray: Of course more arm would help That makes it too easy though! The best stock wheelbase FJ pass that I've ever heard of is of a 10.005, done with a bit of a large motor (1400cc!) Thus the goal is to see a timeslip with a 9 on it... and once I've either hit that or decided it's impossible, then I'll decide to either bail out of the entire project or keep the bike... if I keep it, a 4-6" is the very first thing on my shopping list, followed shortly by a sidewinder...

Thanks everyone! When I think of racing an air-cooled inline four cylinder motor, I thought GS immediately and figured I'd ask to borrow some thinking power from you guys

edit:

If anyone cares for more specific information:

The pistons are stock bore (77mm), stock rods and crank. The bottom end has been proven to stay together at 200hp in turbo setups, and with good rods I've heard of 300+ blower setups without failures.

Head is ported, 30.5/26 (stock was 29/25mm) valves. Wiseco 10.25:1 "street" pistons. Cams are Megacycle 286-00, a mild street grind with .375/.375 lift, 257 duration (at .040). Stock cams are .315/.313 lift and 233/232 duration at .040".

[gmansyz]

Clearence

Hiya Trippi
You are on teh right track. When I was making pistons for cars, I would add .100" to the diameter of the valve for the pocket diameter. For bikes, you can squeeze it down a little and just barely get away with .050". So try for at least .025" radial clearence. .015" is a little tight and can work but remember, when the valve opens, it loves to vibrate too along with expanding. Sorry, but either trade the Wiseco's back for ones with bigger pockets or machine yours larger.
Laters
G

[FJman]

Sigh.

Correct answer: Call wiseco and have them look up the engineering specs on the pistons.

News wasn't good.