When they made the 74 inch shovel into an 80 incher, did they just use a bigger bore? If I found some 80 inch jugs and pistons would it just be a bolt on to a 1975 1200 shovelhead?

 

80 jugs won't bolt onto a 74 incher manufactured in 1975. In 1978 they changed the cases somewhat and the jugs for a 74 and 80 fit the same, the only difference being the bore. However, if you want 80 on your 1975, you're gonna have to bore the original 74" jugs. Someone correct me if I'm wrong on this.

 

How far can I punch out stock 74 inch cylinders? What other mods need to be made? Whose pistons would you use?

 

80 CI 4 1/4 stroke 3 1/2 bore

 

80" cylinders will most certainly fit on a 74" lower end,I just tried it.Bolt patterns are the same. The outside dimension of the 74" and the 80" cylinder sleeves are the same so they must have bored the 74's out to make 80".I would think that .040" would be the max overbore on the 80".Good luck

 

I was googling bore and stroke dimensions and found an archived post from this forum that answers most of my questions.

http://shovelhead.us/forum/archive/i...hp/t-9342.html

Sounds like if I go .060 over on my stock 74 jugs I'll have an 80.

 

.060 over on 74" jugs = stock 80" BUT to get a true 80" motor you also need the 4 1/4" stroke crank . 74" are a shade under 4" stroke ( 3.9688" or 3-31/32" ) .

74" bore is 3 7/16" or 3.4375" , 80" bore is 3 1/2" or 3.5000" or .060+" bigger , some of the older 74 jugs could be bored to .100" over but they were at the limit and thin .

Without the the crank change you'll be gaining less than 2 CI , not worth doing really .

You cannot mix and match pistons between the 2 engines either for a number of reasons .

What exactly are you trying to do ? There's a number of cheap ways to get more bang out of a shovel today over all the twisted stuff we used to have to do .

 

I'm running an '80 inch setup in my stock '71 change the wheels, barrels,rods and pistons no machining to the cases is needed you do have to change the barrel studs though as the flanges are thicker

 

I have to agree with Twizted Biker, there are better ways to get more horsepower of of your motor. I had a guy clean up my heads a little (find some one who knows what they're doing) new pistons, rings, cam and pipe, re-jeted my s&s. Now I call my ole shovel the "EVO EATER"

 

I know that the 74 inch cylinders can be punched to .070 over for a fact .... they said that is max .. mine are being done now that's the only reason I know for sure.... I have a 77 shovelhead on a 55 frame .... the lower has 80 in flywheel in the 74 in case, and the machine work is being done to punch out to .070.... what does that make the engine size ....will the heads from that fit 80 in jugs and will those jugs fit the 74 in case ?...just getting started ... my uncle gave me the bike and im working on it ... with what he already had done to this thing it was completely freakin nasty. ! im a rookie on a BEAST !