HarleyTechTalk

Technical Forums => Knuck/Flat Head => Topic started by: JamLazyAss on June 05, 2021, 10:44:37 AM

Title: Flathead Heads
Post by: JamLazyAss on June 05, 2021, 10:44:37 AM
Anyone know if these heads are for 45" or 74" ?

Title: Re: Flathead Heads
Post by: PC_Hater on June 06, 2021, 01:47:15 AM
In that bottom pic you can see the circle from the bore.
Measure it.
2.75" they are 45.
3 and 5/16" 74 flathead.
3 and 7/16" 80 flathead.

Those heads look old and the bore mark and valve cut-outs look big to me.
I vote 74" flathead but I have never seen 74" parts, only 45s. Not many big flatheads in the UK.
Title: Re: Flathead Heads
Post by: JamLazyAss on June 07, 2021, 06:21:26 PM
If that black circle is the bore, then a quick measurement put it at roughly 3 1/2 inches.
Title: Re: Flathead Heads
Post by: PC_Hater on June 08, 2021, 12:46:15 AM
They are 74" heads. The 9 bolt heads were first used in 1935.

I've been having a look at my bike and in my library...
45 heads have 8 head bolts, those have 9.
The advertised bore was 3 and 7/16" but the actual bore was 3.424".
Title: Re: Flathead Heads
Post by: JamLazyAss on June 08, 2021, 09:15:58 AM
Sweet.
I'm thinking there's not too many of these around.

JC:
Why 74 and not 80?
Title: Re: Flathead Heads
Post by: PC_Hater on June 08, 2021, 03:20:37 PM
Your measurement of the circle the marks the bore.
About 3.5 inches is nicely close to actual 3.424". And you are right, they just might be off an 80". I will have to  recheck my sources, one of the books might have a typo!
Title: Re: Flathead Heads
Post by: PC_Hater on June 08, 2021, 04:54:15 PM
OK.
Harley changed the bore and stroke on the 74s from the 1930 3 and 7/16" bore and 4" stroke to 3 and 5/16" bore and 4 and 9/32" stroke in 1937.
The 80s up to 1937 had the same 3 and 7/16" bore but with a longer 4.25" stroke.
In 1937 the 80s got the same even longer stroke but kept the 3 and 7/16" bore.

1930 to 1937 are VL and VLH models, 1937-on are the UL and ULH.

You will need somebody that knows their stuff to work exactly which model 74 or 80 flathead those heads fit!