heavy85 Posted October 30, 2004 Share Posted October 30, 2004 I posted this on the 6 cyl board since it's for an L6 but didn't get a response. It seems like there are some knowledgeable people over here and I dont think it's specific to Datsun engines so I'll try here. Here it is: I just got done porting my P79 head (well just gasket match and smoothed out the casting marks) and am starting in on the valves. I've searched but not come up with any specifics other than 'polish valves' so what should be the surface finish on the stem, the top (port) side and the bottom (chamber) side - I'm thinking there may be a difference but dont know? I found I can get a mirror finish by stepping through several grits of sandpaper and ending with metal polish but where should I stop? Also, there is a chevy bow tie-ish mark cast onto the bottom side of the valves -should I grind this off to make is completely smooth or just leave it? The reason I started in on the head was from the carbon build-up on the valves in the first place so I'm mostly trying to reduce that in the future but any performance gains would also be nice. The head had already been shaved and it's got a Schneider cam (I think), tripple Mikuni's, MSD, header, etc ... Thanks Cameron Quote Link to comment Share on other sites More sharing options...
Guest Magnum Rockwilder Posted October 30, 2004 Share Posted October 30, 2004 The stems should have a slick polished finish, but you have to be careful not to take off any material or you'll have a valve stem seal leak. The backside of the intake valves are typically finished with a slightly swirled surface to promote a tumble as the air enters the combustion chamber. The backside of the exhaust valves can be polished, but it won't gain you much, if anything. The faces of the valves can all be polished smooth, and you can grind off the lettering if you care to balance the valves (a digital scale is necessary). You can also smooth any sharp edges in the combustion chambers and polish the surface. This prevents hot spots and can decrease detonation. On anything short of an all-out race motor, this is all nearly pointless. Just clean everything and install it and you'll never know the difference. Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted November 1, 2004 Share Posted November 1, 2004 Well, I have to bite on this one. Valve seat face needs to seal, period end of story, that is why quality machine shops do a vacuum test on rebuilt heads. Typically at least a 300 grit stone will get you there, that is what most valve resurfacers have if not finer. Lapping valve in is recommended for valve seats done with the old "stone's". When a valve opens and closes several thousand times it literally beats the seat and valve so that it is mirror like in appearance. Backcutting the valve face in the exhaust will net you a 10HP gain in a v8 if done right, since it has to complement the valve seat angles. Radiusing the valve seat between the seat and the bottom of the valve is also a good idea, good for another 2- 4 HP. But if your looking for horsepower there then you have already done every other trick in the book and you'll never know or feel the difference anyway. Ol' Magnum has it right. Swirl polished heads are great for race engines, but with carbon buildup on street engines, the benefits don't last long. The actual stem is usually chrome plated for wear resistance. If they have more than .001 wear, junk 'em and go new or they'll suck oil, carbon up and wear out the seats so fast it'll make you wish you did it right the first time. Never, never touch a stem with anything that removes metal. So that leaves you using a rag. Quote Link to comment Share on other sites More sharing options...
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