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This is a question for you Ironhead Sportster guys.. I am trying to figure out what year the U-shaped seat mount area pictured below changed from a cast style (see 2nd pic) to this square bar (approx 5/8" x 1") style in the first photo. Some time in the early 70's, but trying to nail down a year.



'Straight Bar' Style Seat / Shock Mount



Forged Style Seat / Shock Mount
 

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I'm going with 1972 as the changeover year. I've had both 71 and 72 and the 71 had the cast seat section where as my 72 had the stamped bar.

Also the neck changed in 72 from the earlier round one to the square neck. I've never seen a stamped bar seat post with a round neck and I've never seen a cast seat post with a square neck.

1972 was the first year 1000. Stamped bar for 1000cc, cast seat post for 900cc
 

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Do you know the exact year of make on the 1st pic? And the "support" braces coming down from the back of the u what's with those? I am having some difficulty with a similar frame pics posted on the board
 

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Forgot exactly where I got this info but I believe it's on the money.

"In 1972 Harley Davidson modified the Sportster frame whereby the cast steel connection node at the top rear of the frame (i.e. underneath the saddle) was replaced by a U shaped stamped and welded connection. While this frame design change took place in 1972, worldwide delivery of Sportsters with the old frame lasted until well into 1973.

The stamped and welded U shaped connection underneath the saddle was not a big success, to say the least. A few months after its introduction, the stamped and welded U shaped connection appeared to be not strong enough to withstand the dynamic and cyclic vertical loading introduced during riding by the rear shocks and heavy rear fender.

This forced the Motor Company to recall all newly delivered Sportsters back to the dealers who were provided with steel reinforcement strips that had to be welded in place diagonally to strengthen the support points of the rear shock absorber and rear fender strut. "

"1973 was a change in the design of the frame.
1972 and earlier used a heavy rough casting where the top tube and the vertical rear tubes meet.

1973 did away with the casting and used what looks like lightweight welded strap metal with an angular bracing running from the rear engine mount up to the end of the fender supports.

The 1972 and earlier frame tubes are canted at an angle of 10° where the 1973 frame tubes are at an angle of 9°45'. Not a great difference but still different.

But a greater change (apart from the casting elimination) is the distance between the head tube and the rear vertical tubes.
1972= 28.42 inches
1973= 29.04 inches
Even though the 1972 frame head had a greater rake angle (30°) the 1973 rake angle was less (29°35') the distance to the rear tubes is greater."
 
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