When talking about stock HD evo pistons, I would like to be enlightened about the existence of value relief or flat tops and clearance or the lack of...

 

"Flat tops" as descriptor would also include flat topped pistons with valve relief cut outs in them. I think you are confusing "flat topped" for meaning only absolutely flat topped (i.e. no cut outs).

Early Evo pistons did not have valve reliefs cut out in them but my guess would be pretty much all of those will have been swopped out by now. Possibly a '86 might not, have you checked yours?

Obviously you understand that the idea of the cut outs is to give the valves additional clearance so that they do not hit the pistons when they are wide open at TDC. This matters more when you put in a higher lift camshaft and/or bigger valves.

You cannot use fully domed pistons in an Evo because of the D shaped combustions chamber, however, you can use partially domed high compression pistons designed especially for them. Those would then tend to have even bigger cut outs providing allowances not just for the reduction in clearance but also for higher performance cams and valves ... based on the theory that anyone intentionally increasing the compression ratio of the engine is probably using a higher lift cam and/or bigger valves too.

From an engine design point of view, flat top pistons generally allow for better combustion (better flame propagation and faster/fuller burn characteristics). Shovelheads, by comparison, had hemi-spherical combustion chambers and so the tops of the pistons would be fully domed in order to fill them, and to increase compression.

The next questions then are, "does my engine I actually need cut outs?" or "are there any benefits to cut outs?".What might necessitate requiring cut outs?

If you skimmed the head, reduced the squish (e.g. by using a thinner head/base gasket), or stuck in a higher lift cam etc.

What are you thinking about doing?

Why did H-D change from absolutely flat topped to flat tops with cut out?

I don't know. My guess would be because it would allow for more variances (safety margin) during manufacture, making assembly cheaper by not requiring every engine to be measured up.

Just for the sake of information, from those with a wide range of experience with Evo engines, I'd be interested to know what the tolerances or clearances are in general.

 

That's some good stuff right there...
My pistons are original, no relief, stamped "84" on top...
I am installing an ev27 cam ,thinner head gasket, (.045) ,no skimming at all...
Just curious about relief and such and appreciate the schooling, I know there won't be any clearance issues with this. Should be squished to .042". That being said, no relief is probably why it came with a .060" gasket...

 

So, you've removed your heads to fit the new cam and measured how far up/down in the hole your pistons are, e.g. + 3 thou, to work out the squish?

How much of your money can we spend?

If your heads are off, and you obviously know what you are doing, I'd measure the tolerance up just to be sure using some modeling clay.

What thickness of base gasket have you used?

If tolerances are close, and you have to consider new pistons with reliefs, folks are going to encourage you to use ones higher compression ones to get the best out of that cam.

 

you must change the pistons as they do not have exhaust valve pockets - the cam lift of the ev 27 the valves will contact the pistons when you start it up not if they will they will

 

Neo-con, I have a threat already going on my rebuild "evo rebuild under way" if you would like to be up to speed on that.
John, no one yet has mentioned that going to a .045 from an.060 gasket will require new pistons with reliefs,what's up with that?
Everyone says ev27 is a swap out cam but never any mention of,"only if you have valve relief" .
Shame on James gaskets for giving a so called stock gasket in there kit when .060 is actually stock without a piston swap !!! WTF !!!
More home work to do I guess.Thanks

 

Glidein wide, you need to listen to John... if your pistons don't have valve relief cut outs and you swap in an EV27 you're in for a world of hurt.

Nice return Lucky Luke...

 

do your self a favor you have pistons that were a mistake by harley - the original lift of that weird can it came with has 416 lift the 27 is at least 450 i dont remember off the top of my head but close - the exhaust valve when the lifter pumps up will crash into the piston seen one actually brake off a valve guide back then

i would bore the cylinders .010 and get a cast set of stock type flat tops then sell the .042 and that is the stock thickness not .060 a reason is in the mix why they use them but we are not in europe - TUV emissions

the cats *** with the 27 is an .030 head gasket it may not sound like a lot but its good for 4 HP in the machine just that addition

 

The 27 has a 495 lift.

 

i have done this year 17 old big twins - some were knuckels, pan, shovels, flatheads and some chiefs no evos in a year my head numbers is 30 plus years back