Topic: Primary Drive Rebuild

[kmb69]

I was working on installing new rubber dampers in a forum brother's primary drive this afternoon until I discovered something that made me come to a screeching halt. I have already done a couple of these for myself except I was converting them for a HyVo chain and installing better dampers and now I am planning to redo them. As I was working on his, I thought the m6-1.0 tap was just going too easy and not making the expected amount of shavings. So after tapping the first 4 holes, I stopped to check the hole diameter. It is 5.2mm where the proper minor diameter for a m6 is 5mm. That's roughly 20% less thread engagement than a m6 should have. It is also larger than the minor diameter for a 1/4-20 not to mention the 1/4-20 or even a 1/4-28 has a 82 degree countersink versus the 90 degree on a metric screw. Probably be OK loctited in for a street motor but I would be somewhat concerned for a performance motor. Since I already machined the side plates with 90 degree countersinks, I plan to get some m6 heli-coils Tuesday to get better thread engagement.

[madmtnmotors]

Good information indeed, thanks for sharing. I will be redoing one or more primary drives in the near future as well. 

[scottly]

Minimum minor diameter for 1/4-20 is .196, or 4.978 mm, and max is .207", or 5.257 mm. Minimum minor diameter for 1/4-28 is .211", or 5.359 mm, max is .220", or 5.588 mm.

[kmb69]

Minimum minor diameter for 1/4-20 is .196, or 4.978 mm, and max is .207", or 5.257 mm. Minimum minor diameter for 1/4-28 is .211", or 5.359 mm, max is .220", or 5.588 mm.

Yes, there is a tolerance range, but I always use a #7 or .201" for 1/4-20's. And sometimes 13/64" or .203" for hard materials but this is aluminum and kinda soft at that. It would be good for a 1/4-28 but I have already machined the side plates to the correct depth for 90 degree countersinks. Plus, I don't like to put Imperial threads in my metric toys. The guys selling the kits claimed the hole to be 5mm and I just took it for granted they knew. Shame on me. 

Last year, I bought a lightened, race ready, Automatic crankshaft along with the converted 5 speed HyVo primary from a guy in the Netherlands. It had replacement 5.2mm flat head rivets very similar to the originals but I have not been able to source them. Replacement rivets would be the best solution in my opinion since it would leave the original strength in the hub. Honda obviously had no intention of rebuilding these primaries.

[scottly]

Rather than helicoils, could you fit M8 1 bolts?

[kmb69]

Rather than helicoils, could you fit M8 1 bolts?

The head on M8's is way too big. I considered using M7-1.0 but I could not readily source them either. Pretty much requires flat head socket screws for clearance, especially on the bearing side. Also going to look for 5.5mm rivets and drill the hub if I can find them. Could probably use 6mm or 1/4" rivets as well.

I might try to make some 6mm rivets from some not so hard M6 screws. The original rivets are pretty soft - mild steel.

[scottly]

Where is the head too big, diameter or depth?

[kmb69]

Where is the head too big, diameter or depth?

Both and 8mm starts to thin the hub. I'm backing off the helicoil idea for the same reason. 6mm rivets will leave more material on the hub. Just have to machine the head on some M6 screws and prep the other end for riveting in the 90 degree countersinks. It will not take much flaring to get them tight and prevent movement. I may have to go 1/4" since the side plates are already machined for the M6 screws but maybe not. The original rivets come out a little bent so they were not press fit to start with but they are riveted tight enough the side plates don't move. Hell, it might be good enough to just use red Loctite and let it be. That would be the easiest fix. Maybe I'm over thinking this. 

If the forum member this primary belongs to is reading this thread, he's probably saying, "Holy $hit, what's he doing to my primary? This was suppose to be simple!" 

[scottly]

Maybe I'm over thinking this. 

This is quite possibly the case. What is the depth of thread engagement with the M6 screws? 

[PeWe]

The german guys use rivets with their kit. They have done 2 primary drives for me, I sent the drives to them for the job just for sure.
http://www.fourever-classic-parts.de/index.php?option=com_content&view=article&id=26&Itemid=31

[cbr954]

Cycle X is rebuilding these now also and then balancing them.  May be worth calling Kenny and get his take on thread situation?

[kmb69]

This is quite possibly the case. What is the depth of thread engagement with the M6 screws? 

Approximately 0.0157" per side where approximately 0.0197" is nominal.

The german guys use rivets with their kit. They have done 2 primary drives for me, I sent the drives to them for the job just for sure.
http://www.fourever-classic-parts.de/index.php?option=com_content&view=article&id=26&Itemid=31

Thanks for the link PeWe. Riveting is the best way IMHO.

Cycle X is rebuilding these now also and then balancing them.  May be worth calling Kenny and get his take on thread situation?

Just talked to Kenny about this 2 weeks ago. It appears he is using 1/4-20 screws which is better than the M6 IMHO. I did not realize at the time the hole was 5.2mm.

[Bill/BentON Racing]

I agree, but then it ain't metric! Lol. Honda doesn't  use much 7mm stuff, CB450 had 7mm chain adjuster bolts....but big heads.

[kmb69]

I agree, but then it ain't metric! Lol .....

Yeah and that probably doesn't make any difference since this is not something you want to be working on in the pits anyway! 

[kmb69]

I'm going to see if the M6's torque to the specified values and go from there.

[scottly]

What I meant was the length of the thread engagement? The more threads, the better.

[kmb69]

What I meant was the length of the thread engagement? The more threads, the better.

There is about 11-12mm of thread engagement. Nearly twice the diameter or 11-12 times the pitch. That does sound better doesn't it.