This subject has come up before and the answers are somewhat vague. Apparently Moore’s Cycle and SRM make kits for this- however being a machinist I always want to make my own stuff so I plunged into the nuances and befuddlements of this adaptation.
To be clear this is a '70 TR6 frame and a '71 or '72 front steering head. It has the big scoop drum brake. There may be slight differences with other years of both frames and front ends.
(I actually "dual pathed" this whole scenario- I first tried to use the original OIF steering stem bearings with my own sleeves to fit the '70 frame- I'll get into why this didn't work in a bit.)
Okay let's look at the steering stem- Triumph couldn't have created a more difficult situation in regards to the stem and the changes they made when they went from the earlier frames to the OIF:
On the left is the steering head stem and lower clamp that typically fits a pre-OIF frame. On the right is the OIF steering head stem and lower clamp.
On the pre-OIF the stem measures about .994" until you get to the step at the base of the stem where you see some braze- there it jumps to the larger diameter of 1.064". The I.D. of the bottom bearing in the Lowbrow Customs tapered bearing kit (For converting the unit frame to tapered roller bearings) is 1.0625" so it is about a .0015" press fit on this slightly larger diameter.
On the OIF stem the stem at the bearing locations measures exactly .750". The step at the base of the OIF stem measures 1.051"- roughly .013" smaller than the pre-OIF! So you cannot get a good register fit of the bearing I.D. on this OIF step because it is simply too small!
The bearing is around .575" in length, the step on the OIF is about .250" in length so you can make a spacer that is 1.0625" O.D., .750" I.D. and .325" long that when slipped into the bearing favoring the top half or the smaller end of the bearings taper- it will now allow it to fit flush with the base of the stem clamp and will register on the original bearing surface making it a good solid- registered fit.
(I happened to have some A-2 tool steel that was exactly 1.0625" OD- with a bit of polishing it slips into the bearing snuggly- I then drilled and bored it for the .750" I.D. so that it fits the stem nicely. You could use any steel- but A-2, O-1 and drill rod is available in the exact O.D. that you need and typically comes with an almost ground like surface on the O.D.)
I think the photos are self explanatory:
So now we have the lower bearing properly fit to the OIF steering stem.
Next is the top bearing situation.
The Lowbrow bearing kit's top bearing is identical to the bottom bearing dimensionally except it had a dust shield swedged into it but only part way:
In this photo I already have the spacer slipped into it up to where the dust shield extends into the bearing I.D. I made this spacer identical to the bottom one except I made it about .500" in length to be sure it would register on the OIF stems bearing surface.
The OIF stem nut slips nicely into the swedged dust shield and will come up against the spacer I have fitted to the I.D. of the bearing.
Okay to recap- we now have spacers that allow the Lowbrow bearings to properly register and fit on our OIF stem- the hard parts done! Once you install the races into the frame as per the usual methods as shown in English 101 and so forth you can simply grease your bearings, slip the lower one onto the stem, slide the stem up through the steering head and install the top bearing with the dust shield- then the top clamp then the steering stem nut and snug it all down and Bob's your uncle!
Except for one thing- the lower clamp on the OIF is a bit different in that the little lip that leads up to the machined lower bearing surface is "taller" and will interfere with the frames steering head and not allow the bearing to seat properly:
In this photo you can see a shiny spot and a burr where I milled some material away on the vertical lip coming from the bolt hole area of the forging/casting. I in fact need to do it a bit more. Study the two clamps in this area- I think you'll see the differences:
Once this is properly clearance everything should fit up sweetly! The original fork stops will work too.
The one thing I'm going to do is make an aluminum or possibly a steel tube to slip over the stem to be captured between the two spacers- why? To make sure that they don't slip out of the bearings to the middle of the stem. Now this get's a bit tricky because you can't measure exactly what the distance between the two is once it's installed. But you can install it all with out the tube spacer and measure how much your steering stem extends above the dust shield on the top. you have to hold it tightly in place and then measure- mine was roughly 5/16". Pull it out of the steering head, mock it up on the bench and measure between the two bearing spacers inside to inside on the stem- this will get you close- I'd make it a bit shy of this over all length- the main thing is that the spacers never slip down or up completely out of the bearings- this way you keep them in place- make sense? If not I can photo graph this as well. Just make sure everything tightens up properly and there is no vertical play in the steering head assembly once it is installed.
There you have it- or at least what I came up with!
Okay so now why can't we use the OIF's original .750" I.D. bearings?
I tried this first- got some new bearings and races. (Which in a standard size like that are really inexpensive I might add!) I then went to the work of actually making the sleeve to press onto the races that will allow it to fit the pre OIF frames steering head. But before I made the sleeve I took this much smaller race and held it in the area that it should seat on in the steering head- and as I feared it really didn't sit properly due to the radius on the outside bottom edge of the race and the flat in the steering head which was just narrow enough I was afraid the race could possibly pound into the portion of the steering head below the race flats. I hope this makes sense- I can do a drawing to clarify. I thought "no problem"- I'll just make a "cup" with a bottom that will allow it to seat properly in the steering heads race pocket. Here is what I came up with:
Sweet! So I installed these into the steering head and slipped the OIF stem and lower clamp in place and slapped the top clamp in place and... The stem just didn't come up far enough that I felt like I would get the stem nut adequately threaded on there.
If you look closely these smaller bearings are Taller. Plus the OIF bearing sits on top of the larger O.D. part at the base of the stem. That along with the fact that I had to make a pocket style sleeve (even though the bottom thin flange in my pockets is only about .040") the multiple stack ups was just too much and my stem came up short.
So in the end I went back to the early tapered roller bearing conversion kit and made the "internal" spacers- this gave me adequate stem extension out of the top to get the nut threaded on good. You might be able to find shorter .750" I.D. bearings... but they get expensive... so I went the other way and I believe it will work out much better.
I hope this helps others making this conversion in the future.
Do you know if the Lowbrow Customs tapered bearing kit (For converting the unit frame to tapered roller bearings) will fit a set of pre-unit forks. They bottom tree looks like the one you have pictured on the left except my forks have these two "wings" on them. I wanted to order that set from lowbrow but I'm not sure if they will fit my forks. Any advice would be greatly appreciated.
Last edited by Howitzer; 04-05-2010 at 12:43 AM.
now, if you could just do this for a 69 BSA neck...i would think you're totally killer.
i mean this is already an awesome thread with tons of valuable info...
but i don't really feel like paying over $100 to get some tapered roller bearings from somewhere in the UK.
tri and bsa front ends look almost identical except for the stem size..i think BSA stems are larger(?)
i've sourced tapered bearings from some stores overseas...
and it's like $90 for the bearings itself...but then they bite you in the but with the shipping.
oh well...gotta do what you really gotta do sometimes i guess.
just want to make my bike right and good.
Last edited by Howitzer; 04-11-2010 at 4:42 PM.