Piston to valve Clearance?

gvr4 · 12-29-2012

Putting together my 6-bolt with evo-9 pistons and its looking like piston to valve clearance is pretty tight, I'm talking something like .03 intake and .05" exhaust. (I have to bring my indicator home from work and get some real numbers).

Its almost common knowledge that a pushrod engine needs .08" intake and .100" exhaust, but I haven't even seen any recommendations for an OHC engine...

Any engine builders out there have a rule of thumb for piston to valve clearance? My build is nothing fancy and won't be spun higher than 6500 on purpose.

sleepydsm · 12-30-2012

Wait.... 0.030" and 0.050" sounds like a lot to me. I as long as it's not zero does it really matter? lol.

I'm a bit biased because I work with things in 0.0001"-0.00001" range

gvr4 · 12-30-2012

Do you deal with metals at those tolerances? If so, I'm sure you are aware of the changes that will occur over the temperature range seen by a component like a valve. 0 clearance at 70 degrees means -.025" or so at 600-700 degrees

There's also a phenomenon called valve float that really tends to make a mess out of things if there's too little clearance. It's easy to see how a pushrod engine would end up with quite a bit of valve float because of all the extra mass in the valve train. But valve springs in an overhead cam engine have a lot less inertia to contend with, and I'm wondering if this translates to tighter acceptable clearances.

gvr4 · 01-02-2013

Called DSM Graveyard and they said they've run them with as little as .025" without issue. Just figured I'd bump this in case someone comes across this thread in a search. Actual numbers on this topic are pretty scarce.

One other thing to consider; if head and block have been milled enough to cause concern with piston to valve clearance, the cams should degreed in with adjustable cam gears since at that point the timing belt geometry has changed significantly.

**Halon** · 01-02-2013

I've never actually measured any of that. These are interference engines so if you have valve float then no matter the tolerance, you will run the risk of valves touching pistons. You shouldn't be seeing valve float at 6500rpms though, unless you have some insanely old wore out springs, and/or some very aggressive cams.

Unless both the block and head have been milled several times, or you're running some sort of unique or crazy setup, I guess I wouldn't be overly concerned because it is what it is.

gvr4 · 01-02-2013

I guess I'm just a little anal when I build my engines. I'd rather check it now than find out the hard way O.o

turbotalon1g · 01-02-2013

I'd talk to guys that build motors, MAP, DB, Gosh, a few users on here could chime in too.
This place is bolt-on central these days though.

gvr4 · 01-02-2013

Well... The executive decision has been made! f**k it, run it.

.027 measured P2V.
.052" uncompressed HG will compress to .051"
Leaving .026" PTV clearance.

Its about time I finally get to finish putting this thing together.

**Halon** · 01-02-2013

Being that you're anal, what's your squish pad measure in at out of curiosity?

gvr4 · 01-02-2013

Haha don't worry bout it.....

.055" in the hole + hg thickness

**Halon** · 01-02-2013

.055" deck height + .051" HG? Dayum.

gvr4 · 01-02-2013

Yeah. I'm not sure where the difference comes in... Evo 9's have a different deck height? Rod length? Or wrist pin height? Idk. Not sure if its worth the $600 it saved me over going with a MAP short block either...

**goodhart** · 01-02-2013

Sounds about right according to your thread here Brandon. Or at least not out of the ordinary for a DSM engine running Evo IX pistons.

http://www.mitsustyle.com/forums/showthread.php?t=32088

gvr4 · 01-02-2013

Quench really comes into play on NA engines, but I would think that with the dynamic compression these mitsu engines are seeing, it doesn't have as much of an effect. Especially with dish pistons, there really isn't much quench area on the DSM pistons I've seen, whereas on my Trans Am, almost half the piston is quench area. On my Trans Am, with a -.001 deck and a thin gasket, quench height is at .032". Its details like that that let me run a nice streetable cam and still make the same power as guys running xe284's.

I'm still kinda shaking my head on how little actual information there is out there about these engines. I've been unable to find the stock dish cc on the IX pistons, but I finally did come across the number for milling the heads: 1cc per .007" milled. It would be nice if someone would take the time to publish all the numbers for various configurations, since basically you can mix and match parts from any of them to get OEM parts for any goals you have.

sleepydsm · 01-02-2013

Do you have have gray hair?

gvr4 · 01-02-2013

Quote:

Originally Posted by sleepydsm

Do you have have gray hair?

No.

Where do you work that your measuring parts down to 10 millionths?

**Halon** · 01-02-2013

Quote:

Originally Posted by gvr4

Quench really comes into play on NA engines, but I would think that with the dynamic compression these mitsu engines are seeing, it doesn't have as much of an effect. Especially with dish pistons, there really isn't much quench area on the DSM pistons I've seen, whereas on my Trans Am, almost half the piston is quench area. On my Trans Am, with a -.001 deck and a thin gasket, quench height is at .032". Its details like that that let me run a nice streetable cam and still make the same power as guys running xe284's.

I'm still kinda shaking my head on how little actual information there is out there about these engines. I've been unable to find the stock dish cc on the IX pistons, but I finally did come across the number for milling the heads: 1cc per .007" milled. It would be nice if someone would take the time to publish all the numbers for various configurations, since basically you can mix and match parts from any of them to get OEM parts for any goals you have.

Magnus offers a piston with "an improved" quench design for the 4g63.

Other manufacturers design boosted engines with a more ideal quench band than mitsu does. I'm curious why others care and mitsu doesn't.

Did you measure deck height with the stock piston by chance? My notes were a 6 bolt would be closer to .015, but not sure how accurate that is. If so that means the evo 9 piston sits way lower.

Goodhart, yeah I agree his hg measurement is what id expect according to my post, but the .05 deck height surprised me. But the evo 9 deck height was around that, so I guess that just means these piston are that much shorter maybe. Kinda interesting.

gvr4 · 01-03-2013

There could be a number of places that deck height difference could come in. Actual deck height (crank centerline to gasket surface), rod length (doubtful but who knows?), or compression height (wrist pin centerline to quench pad).

I didn't measure my old rotating assembly, but IIRC, it wasn't more than .02" in the hole.

Pushit2.0 · 01-05-2013

At what point are you measuring PTV? To get an accurate number the motor would need to assembled and rotated over, I would have to look at my paper work but I think ptv will be closest on the exhaust stroke as the piston comes up on TDC.

The evo9 piston will be a 22mm wrist pin, did you have 1g rods machined for the bigger wrist pin? Or are they evo 9 rods?

If the piston is down the hole a little that's fine. When it sticks out is when you can have issues. Also what size valves, valve springs/ retainers, and what cams/cam gears?

I want to say my race motor was somewhere around .025" ptv on the exhaust side and I would shift at 10,000rpm.

gvr4 · 01-05-2013

The rods were machined for the bigger wrist pin.

The reason it's my intake valves that are so close is that the evo valve reliefs are backwards compared to the dsm, so the exhaust is sitting in the intake valve relief, which is fine, but the other valve relief is smaller, so the intake valve can't fit inside its relief. So normally, you're right, the closest point would be when the piston is chasing the exhaust valve closed on the exhaust stroke.

What's your valvetrain like in your race motor? I'm running all stock stuff, so I think I'll keep my revs a little lower with that tight of clearance.