Can Anyone Say HEMI?

[Jeff Higgins]

I thought you guys might get a kick out of these photos. I was recently presented with the "opportunity" to dig back into my '76 "Ironhead" Sportster that I rebuilt a couple of years ago. The darn thing broke the exhaust valve spring on the front cylinder, the valve hit the piston, and the head of the valve broke off. Made a bit of a mess...

I wound up having to replace the cylinder, piston, both valves, both springs, and the guides on the front cylinder. The head and rocker box, miraculously, survived. An inspection of the rear cylinder and head revealed a broken exhaust spring there as well, so all springs have now been replaced. With a different brand, by the way.

So here is what a real old school, 1950's era technology "hemi" head looks like. It's hard to capture in a photo just how deep the dome really is in these things. That massive dome on the piston only brings compression up to 10:1. Talk about dated technology... the valves and ports are massive on this thing, considering its small displacement and relatively low rev limit. My, how times have changed.





Oh, and check this out. Dan Baisley's own, in-house roller tipped rocker arms. These extend valve guide life tremendously in the old Harleys by eliminating any side loads between the valve stem and guide:



I should have this all buttoned back up tomorrow. Nice little rainy Saturday project.

[onewhippedpuppy]

That looks like serious fun.

[herr_oberst]

10 to 1 on pump gas? Is that stock or did you bump it up? If you bumped it up what sort of ignition upgrades do you do to keep it from detonating?

[Rednine11]

10:1 wont detonate on pump gas with Hemi heads

[Jeff Higgins]

Originally Posted by herr_oberst

10 to 1 on pump gas? Is that stock or did you bump it up? If you bumped it up what sort of ignition upgrades do you do to keep it from detonating?

Stock is 9:1, so yes, I did bump it up a point. The only ignition upgrades are a breakerless replacement for the points and a high energy coil. Timing is set at 35 degrees total, coming in at 1,800 rpm. You essentially don't run these things much below 3,000 rpm, so it's really running at full advance if it's running at all. It retards to about 10 degrees at idle, and to make it easier to kick.

Some guys like to twin plug these things. I'm a big believer in twin plugging our 911 motors (my 3.0 liter MFI motor runs an Electromotive crank fired twin plug setup). Old Harleys are a different animal, though. Extremely undersquare motors - this one is 3 3/16" bore by 3 13/16" stroke. So, a second plug would not be all that far away from the first one, and the first one gets exposed to the whole combustion chamber relatively early on the (long) combustion stroke. There really does not seem to be any advantage to twin plugging a street motor in this compression range. Kinda counter-intuitive to everything we "know" from our 911 motors.

[livi]

Thats cool stuff.

[dennis in se pa]

That is beautiful.

[asphaltgambler]

Twin plugging an iron motor is a must in my experience. The combustion chamber is.........like...cut-in-half a grapefruit. Valve angles all wrong and said above ports too big for that size engine even though long stroke engines require much more runner volume for the same displacement.

Duel plugging these heads require little extra expense in machining and ignition parts. The gain in Hp/Tq comes with decreased total timing and fuel curve.

[John Rogers]

I had a 1963 Sportster and it had about 11 to 1 compression but with the Sifton minus minus cams they had so much over lap that it was never very high except at very high RPMS. The heads and pistons looked about like yours do. Oh yeah, it had a magneto, off of a John Deer tractor with an adapter shaft if I remember correctly but it was a long time ago??!!