Question regarding 10.5:1 pistons and 1200S Lightning heads

[rottenralph]

you will be at 400 bucks for a proper bore and good pistons and a 1250 will cost about $700. May just be a good time to get that 4% bump. I am toying with a 1250 with the high comp pistons that fit the XB chamber. I am certain that is the route I am taking when my next rebuild is required or desired.

[NRHS Sales]

a 1250 kit is what i would recommend especially if your cylinders are scored up.

Whether to go with a stock style 10 degree piston or a 15 degree really depends on how much work you do to the heads and what compresion ratio you are looking for. if sticking with the stock cams 10.5:1 is too high so the 10 degree would be the better choice even if you do unshroud the valves.

[aswracing]

The stock style piston is a flat top, not a 10 degree. Stock CR is 10:1. Yes, the heads have 10 degree squish bands cast into them, but since HD put flat tops under them, they do nothing.

When you machine the squish bands nice and even and use an angled dome piston, you improve chamber turbulence which helps prevent detonation. That's what lets you bump the compression half a point over stock without causing issues.

Yes, it seems like a big number for W's, but you have to remember, too, that he's dual plugged.

[NRHS Sales]

just a difference of opinion and we all are entitlled to our own. I do not recommend going over 10:1 with stock cams as I would rather give up 1 or 2 hp for the sake of reliabilty since this is not a race bike.

[aswracing]

You have to remember though, that stock squish band in those heads is very rough and uneven and there's an overhang around it. To machine it to 10 degrees typically requires quite a bit of head decking.

Even if he wants 10:1 (which might not be a bad idea considering this was a detonation related failure), I'd still use the 15 degree, and do a valve job or something to get the chamber volume up, or worst case trim the dome. That way you can make a nice even squish band without excessive decking.

[tobiism]

lots of good info from everyone involved, thanks a ton!

[TT60]

Originally Posted by rottenralph

Is there a good example out there of what an unshrouded valve looks like? I am not quite sure why these valves are said to be shrouded.

Here's 62cc Lightning heads with NRHS stage II work with .040" decked and cc's is 64 now. With 10 degree 10.5-1 pistons, 575 cams and .050" head gasket compression is 190 on both cylinders. Going with a .030" headgasket compression is 15 psi more.




OP oem head

[maru]

Nice! Well thought out combo.

[aswracing]

That picture does a great job of showing what a Lightning chamber needs.

What we need is a clean picture of a stock chamber to show how bad they are to start with.