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Home > Technical > Projects > H-D Big Twin round-tube swingarms 1958–72

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H-D Big Twin round-tube swingarms 1958–729765 viewsFor 62-later models I have only seen the forging number below the housing. The same hallmark was often there although I’m not sure how long it remained.

Some late-60s–72 housings have other markings underneath such as 4-H, 4-N, 14-N and HH but I don’t know what they indicate. On top sometimes you’ll see a lone number and these go from 1 through at least 4.

Questions and/or comments welcome regarding any of the info I posted. And if anyone has a swingarm I can help ID then please feel free to contact me anytime.
Speeding Big TwinSep 24, 2016
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H-D Big Twin round-tube swingarms 1958–729764 viewsGetting back to the pivot bolt housing, I do not know if any were made with an EX (experimental) forging number. All the ones I know of bear a forging number that looks like 47556 58 and it can be found below or above the housing for 58–61 models. Usually the forging number is followed by a hallmark but it’s hard to get a clear picture of it. Anyone know for certain what the hallmark is?Speeding Big TwinSep 24, 2016
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H-D Big Twin round-tube swingarms 1958–729764 viewsOutside the left clip is H22 upside down. Notice the lower rib has been increased in length and ends closer to the anchor stud hole than before.

Apparently the increase in length is why the forging number suffix changed from 58A to 58B but exactly when did it happen? I am not sure although it seems to be sometime after 1967 models.
Speeding Big TwinSep 24, 2016
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H-D Big Twin round-tube swingarms 1958–729764 viewsNext change to the left clip forging number was when it became 47614 58B. Hallmark is still A. In this instance the die number is 13.Speeding Big TwinSep 24, 2016
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H-D Big Twin round-tube swingarms 1958–729784 viewsI think clips with that die number (11) may also have been the first to have the boss left undrilled but pictures I have thus far are not very clear.

I do not know when the left die number became 12 but the forging number was 47614-58A. As you can see, the only hole for the brake hose clamp screw is the one toward the rear that seems to have first appeared at some stage during the 1967 model year. The rectangular boss introduced for 63 remains in place but has no hole.
Speeding Big TwinSep 24, 2016
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H-D Big Twin round-tube swingarms 1958–729784 viewsAs another example of pieces not necessarily used in the order in which they were made, some clips with die number 11 still used the brake hose clamp screw hole in the rectangular boss.Speeding Big TwinSep 24, 2016
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H-D Big Twin round-tube swingarms 1958–729785 viewsIt may be that early-67s only had the hole in the rectangular boss first introduced for 63 and my guess is most swingarms for 1967 had that hole tapped ready for use. But at some stage during the 67 model year it seems the factory wanted the hose clamp mounted further to the rear and that may be why some swingarms have two holes. I expect very late 67s only have the rearmost hole, as do 68–72 models. For those swingarms the rectangular boss remained in place but it did not have a hole.

Something else that seems to have changed around this time was the placement of the twin pressure release holes in the pivot bolt housing and you’ll notice the two below are closer together than the holes shown in the black swingarm earlier in this thread.

However, one swingarm I know of only has one release hole although it is in one of these later positions.
Speeding Big TwinSep 24, 2016
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H-D Big Twin round-tube swingarms 1958–729785 viewsAnd here’s another example.Speeding Big TwinSep 24, 2016
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H-D Big Twin round-tube swingarms 1958–729785 views1967: Left forging number is 47614 58A. Hallmark A is present. I am not sure when the left die number changed to 10 but it seems to be around 1967. As per earlier clips, some die-10 clips have only one hole for the brake hose clamp screw but some have an extra hole further to the rear. Also notice below that there is no boss for the additional hole.Speeding Big TwinSep 24, 2016
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H-D Big Twin round-tube swingarms 1958–729786 viewsNext combination is H8 upside down and die number 9 inside.Speeding Big TwinSep 24, 2016
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H-D Big Twin round-tube swingarms 1958–729786 viewsNext combination is H8 upside down and die number 9 inside.Speeding Big TwinSep 24, 2016
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H-D Big Twin round-tube swingarms 1958–729786 viewsBut the die number inside is still 8 as per the previous example.Speeding Big TwinSep 24, 2016
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