This is the spot I'm worried about now. There's no way there is .050" clearance there. The edge of the clay in the pic is the edge of the valve relief. This pic is where the intake valve is .093" from making contact with the piston inside the valve relief from top to bottom.



It's kind of a catch 22 situation. If I put the regular springs in, pushrods, rocker arms, etc. and the head gasket, the gasket won't be compressed to .028" so the valve won't travel the full length into the chamber right? Without the head gasket it travels too far. The way I did it seems most likely to be accurate but I'm sure I'm missing something.

 

I don't think your measurement is accurate since you don't know where the piston was at the point of minimal radial clearance. The valves are angled so that will make a big difference.

Your gasket will not compress that much. Clay it again with regular springs, lifter bottomed out. The headgasket not being compressed can't throw you off more than a few thousands.

 

Before I saw your last post, I went ahead and put the correct springs in and assembled the other items such as lifters, pushrods, rockers, etc. I bolted the head on with no gasket and adjusted the rocker nuts down so that the lifters were completely compressed.

I rotated the crank over two times and then took everything apart. Other than the valve impression going a little further into the clay, the radial clearance comes out the same.

For reference, in the pics I posted, the wide side is right about .050" clearance and the left side is only a couple thousandths. It was difficult to section the clay with it that thin in that area and get a good measurement.

 

Adding the gasket will improve radial but not much. Looks like you will have to notch the pistons. I don't know if it can be done with the pistons installed. I would think the best way is to remove them and take it to a shop. Kinda sucks but it would have been much worse if you had tried running it.

 

Getting the pistons out is easy; only two are in and neither have rings installed.

I was wondering if this was something I could fix by removing material with a carbide bit in the dremel or if it has to be more precise and go the the machine shop? I don't particularly like/trust the local machine shop but I suppose they can't screw this up too bad.

 

Dremel should work. The clearance is the important part. Doesn't matter if it's a little uneven. The amount of material removed is so small there are no balancing issues. Just don't slip and take a chunk out of the skirt!

 

Multiple mock-ups are a necessary PITA, and I'm glad you've shown the discipline to have caught this before firing up. I'd suggest following up on that discipline by taking your heads, pistons, cam specs and findings back to the machine shop and have then perform this surgery.

Sure, you or I could do it with a dremel, but are you after professional quality or shade tree? Personally, I just couldn't bring myself to do it, as I'd be up nights wondering how I may have effected the comparitive CR's, flow characteristics, burn patterns...

And, yes, an incompetent machine shop can screw this up. Have a serious talk with them about this first, and make sure they don't unnecessarily plunge anyfurther than required into your pistons.

 

You could try an Isky Piston Notching Tool (PNT-216, PNT-158). It's a valve shaped cutter that uses the valve guide of your cylinder head as a drill bushing. This could reduce any guesswork from using those twisted wedges. The Isky online catalog says they are even available for a $40 rental fee. Summit has them for $143 apiece (ouch). And I don't think that includes the valve stem dia pilot (not sure).

BTW, an old engine builder around here used to say that you should be able to fit a dime in between p/v at their closest point. So your .093 should be cool.

Also, (correct me if I'm wrong) if you were to use a multi-layer steel gasket, you'd be able to bolt/unbolt the head as many times as you wish.

 

Don't try the Isky tool, years ago I used one on a 340 Mopar, tool quickly dulled and was a PITA to use. I would go with light touch-up with Dremel tool. Shouldn't remove enough material to cause balancing concerns. Use a black sharpie marker to lay out with a scribe the material you need to remove. One other tool you can make is to get a old valve with a old head and braze a 1/16"-1/8" cutter from a machine tool supply house to the head of the valve and use it to clearance your valve pocket (I have used this technique several times over the years with great success.)

 

Boy...I'm glad you checked this!!! You had a valve dropping episode in the making there! Now you see why I tell people to check this..it's easily missed just by concentrating on P/V depth clearance.

The best way I know is to just clay piston, set up valves like you did and roll motor over at least two times. You really don't care exactly when it's closest radially...and it could easily be at a different point than the minimum PV depth measurement. Just roll it over through a couple of revolutions and see what imprint looks like.

Another method that is very precise is to take an old intake valve, cut head off and put it in a valve machine to grind a point on the end. Take all intake valves out of head....bring piston to TDC...and also check at 10* BTDC and 10* ATDC. Then install head and drop centerpunch/valve though guide to mark dead center of valve on piston top. Just tap valve/centerpunch with a light hammer tap. Then remove head and set up a pair of calipers to half of valve diameter. Put point on the centerpunch mark and trace outline of valve diameter around piston top and see exactly where it lies. Then you can use that to open valve pocket. Takes a long time to do this on all 8 pistons...but it's very accurate.

But clay will work just as well....and a lot faster!

Your pics definitely show the issue well. The minimum radial clearance is .050. I personally like a little more for street stuff because we do run them a long time and stuff happens. Maybe a valve guide goes bad....we get excited and over rev it, we miss a gear, we break a driveshaft...we break a valvespring etc etc. I prefer to leave a little extra..just makes me feel good. I would prefter to see .075-.100 raidial.

Now if I'm building a pure race motor..I would go tighter to maximize compression. But race motors don't run long and they get checked a lot more often and they use better parts. We don't have to worry about an extra .5-1 cc in a valve pocket like a race motor would.

The Isky tool will work fine..I've used them before just fine. Not hard to use on engine stand at all and it will definitely put notches where they belong.

I've also just used a dremel tool to do what yours needed. not a big deal either..but if pistons are out of motor, machine shop can set up and do them very quickly. Just take them to him with the clay still on them so he can see what needs to be done. simple stuff.


Sure glad you checked this stuff. Better to be safe.


JIM

 

After several calls to local (and not so local) machine shops, I have decided to get the Isky tool and have a go at fixing this. Oddly enough the biggest machine shop in town doesn't fly-cut pistons and recommended a small shop in the next city over. I couldn't get those guys on the phone. I called another recommended shop in the "big city" and they don't cut pistons either. But at least now I know where I can get dyno tuning later. (Thanks Glen)

The folks at Isky were super friendly to talk with; both the tech guy and the lady that took my order. Isky even rents these tools if you like. Basically you buy the tool outright and when you are done you can send it back and get your money back minus the rental fee which is somewhere around $50 from what I could tell.

So, I'll be posting the results of how it went after I give it a try. The tool should be here on Friday.

 

Use play dough, it is so soft it wont compress the lifter and use an old head gasket so it is already compresss. Also rotate the crank 2 full revolutions so you go through the whole combustion cycle. Than cut the dough with a razor blade down the center ,peel away half and measure carefully. You will be safe with .80 and .100 clearence. Unless your really float the valves. A good rev limiter on the ignition can help prevent that, but no gar-un-tees in this game. Good luck.

I just went back on the post and you said you bottomed out the lifter. You should just adjust them to the proper ,no spin, Tight fit plus 1/4 to 1/2 additional turn. To much compression and the valve will start to open before the proper advance is reached and the distance to the valve will appear to tight. I would retest the clearence before i cut the valve resess in the piston. Just my opinion, but that is what i would do..