Dual-Plug Heads?

[wideglide38]

Are there any minor or major differences in the 74” and 80” heads for Shovels? A friend is looking for 80” dual-plug heads for his ’83 FLHS 80” shovel. He can find several 74” dual-plug heads on ebay; but, none noting 80”. We assumed the combustion chamber might be slightly different and maybe slight valve train differences; but, essentially the same head. His shovel is basically stock except for the new 80” barrels and pistons we installed this past summer. Has anyone run 74” dual-plug heads on an 80” model? His alternate would be for a machine shop to drill/tap his heads.

Thanks,
William

[vph-d]

IIRC, there was no functional difference between 74 and 80 heads. The 80/81 heads had the short valve guides, the mid 78 and later heads had a boss on the pushrod side to mount the bigger air cleaner. I don't remember if there was a difference after the pushrod corks were phased out.
VPH-D

[wideglide38]

Thanks VPH-D, I do recall the early heads having tall fins between the rocker covers. William

[Rubone]

What is his reason for wanting the dual plug heads? Just curious as they are a fine addition on performance engines but of little value on a stock one. The entire concept is flame front and burn time. Hemi heads and high crown pistons (high compression) affect flame travel, so a second plug helps with complete combustion. Is he after performance or bragging rights? The former has some real value, the latter none.

[wideglide38]

Thanks Robbie, I get what you are saying. No, he’s definitely not a bragger. He is hoping to get a little more performance out of his mill (appears not so on a stock mill). And he hasn’t mentioned any plans stroking his mill either. I’m wondering if there are there any gotcha’s to be concerned about (fit, internal clearances, etc.) using 74” dual plug heads on an 80” stock motor?

[Rubone]

No difference in internal dimensions. The later heads just had easier to replace guides than the earlier ones. Cylinder to head connections are identical. Bear in mind that dual plugged engines generally work best with the timing slightly retarded.

[wideglide38]

Thanks Robbie

[shovel_head_fred]

One thing to keep in mind is that some of the earlier heads, not sure of years, used short reach plugs and others used long reach. Make sure of all your clearances before stomping the kicker or hitting the starter button.

[Underdoggie]

I have dual plugs on a 75 FXE and the heads are original to the bike. When I machined down thru the fins to get enough clean up for the plug port, The long reach plugs had a thread or 2 sticking down into the combustion area. After awhile they get a little carbon on them and would tighten up when removing them. So, I use a set of long reach in the left side and short reach on the right side. I can tell you this, I don't think they make any real difference in performance, The engine has 8.25 compression and an older Sifton Cobra solid lifter cam and a 2 into 1 Supertrapp header and an S&S super B carb. It runs real nice for a 74 inch but no screamer for sure.

[ryan]

I have dual plugs on a 75 FXE and the heads are original to the bike. When I machined down thru the fins to get enough clean up for the plug port, The long reach plugs had a thread or 2 sticking down into the combustion area. After awhile they get a little carbon on them and would tighten up when removing them. So, I use a set of long reach in the left side and short reach on the right side. I can tell you this, I don't think they make any real difference in performance, The engine has 8.25 compression and an older Sifton Cobra solid lifter cam and a 2 into 1 Supertrapp header and an S&S super B carb. It runs real nice for a 74 inch but no screamer for sure.

For the short reach plug, do you use a 3/8" or 1/2" reach plug? Also you can buy sparkplug indexing washers. Why not kill 2 birds with one stone and index your long reach plugs while spacing them out a thread or two at the same time. Just my 2 cents.

[T. Cotten]

For the short reach plug, do you use a 3/8" or 1/2" reach plug? Also you can buy sparkplug indexing washers. Why not kill 2 birds with one stone and index your long reach plugs while spacing them out a thread or two at the same time. Just my 2 cents.

Yeah John...

Come get some PEEK scrap and cut your own!

....Cotten

[Shvlbro]

I used 1/2" reach on the right side and 3/4" reach on left side. You can weld up the head to use the 3/4" reach on the right side so all 4 spark plugs are the same reach. I ended up doing this to my 74 cu. in. FXE.

[Shvlbro]

I forgot to mention, later shovel heads on the 80 cu. in. were thicker and could use 3/4" reach plugs on both sides when they were drilled.