Topic: K5 engine in an F3 frame

[andrewk]

Hi all-

I have a K5 engine that I am going to use in my F2 (1977) frame. I am aware that the output shaft of the later engines is different than K6 and prior, but I'm not sure how they differ.  I've been told that they are offset 10mm, but don't have two to measure.

Anyway, I'm using the stock comstar wheels, am I going to be running into chain alignment issues?  Who sells an offset front sprocket?

Thanks for any advice/reply-

[Tugboat]

Yes. The older motors' output shafts offset the front sprocket 10mm out.. so the later frames had a wider swing arm, different sprocket carriers on the rear wheel and little "shoulders" where the LH rear shock mounts at the top.. all to compensate for the front sprocket being 10mm further out. I've done the opposite of what you're trying: late motor in an earlier frame. Not sure you can do it the other way around.. Might need a different swing arm & rear wheel, but then your upper LH rear shock mount is still 10mm too wide. Hmmmm.....

[andrewk]

I did find a F3 bottom end in my parts pile- anyone know if it could be as easy as changing the output shaft?

I was hoping it'd be as easy as either changing the sprocket carrier or getting an offset sprocket... any other old timers want to chime in who have been there, done that?

[madmtnmotors]

The later EARLY motor output shaft is 10mm further INBOARD. If you use the rear wheel sprocket carrier from an earlier model (they are 10mm shorter) then all you need to do is get the rear wheel spacers right.

[andrewk]

Alright, I guess i'm in uncharted waters here or something.

So I read on the cycle-x website about the difference in sprocket carriers, and verified the information with the sprocket carriers I have.  The late model carriers are 10mm taller, which would have to make the output shaft move outboard 10mm,rather than inboard, as this was almost certainly a move for more chain clearance (both for tires and the giant 630 chain they used). The shaft being shorter doesn't make much sense in that regard.

But, I think at any rate, madmtnmotors is right-  use the right sprocket carrier, and it should be just fine.  I'll go do some mock up now and post pics of the result.

I searched this topic (google site:) and didn't come up with much.  Certainly this kind of swap is more common than it seems right?

[andrewk]

Also, wish I wouldn't have put the new bearing and seal in this carrier before knowing if my chain line was going to be straight.  Kicking your own ass hurts! 

[Tugboat]

When I was going through the opposite of the problem you had, I found a lot of misinformation about the swap (see above). PLENTY of guys said they had done it before and that it was just a straight swap, no mods needed.. or just swap the carrier.. or get a wider swing arm.....

In your case, I do believe that you are correct and that it should only require the use of the older carrier and some spacers to make up the difference.

[sethdhawkins]

Please let a brotha know, I'm doing the same swap

[madmtnmotors]

Alright, I guess i'm in uncharted waters here or something.

So I read on the cycle-x website about the difference in sprocket carriers, and verified the information with the sprocket carriers I have.  The late model carriers are 10mm taller, which would have to make the output shaft move outboard 10mm,rather than inboard, as this was almost certainly a move for more chain clearance (both for tires and the giant 630 chain they used). The shaft being shorter doesn't make much sense in that regard.

But, I think at any rate, madmtnmotors is right-  use the right sprocket carrier, and it should be just fine.  I'll go do some mock up now and post pics of the result.

I searched this topic (google site:) and didn't come up with much.  Certainly this kind of swap is more common than it seems right?

A correction is in order, I meant to say the EARLY motors output shafts were 10mm inboard. I misspoke (misstyped). The later motors output shafts are indeed 10mm outboard of the early motors. The rear sproket carriers are the means to keep the front and rear sprokets aligned.

[MRieck]

When I was going through the opposite of the problem you had, I found a lot of misinformation about the swap (see above). PLENTY of guys said they had done it before and that it was just a straight swap, no mods needed.. or just swap the carrier.. or get a wider swing arm.....

In your case, I do believe that you are correct and that it should only require the use of the older carrier and some spacers to make up the difference.

  Better be careful who is saying what on the internet. Tugboat knows my solution which is perfect for F2 engines to F1 frames. That includes the F1 with a JMC (RIF'ngP) swingarm). I am in the process of making 17 teeth countersprockets for the swap. 5 JT sprockets were ordered today.

[andrewk]

Alright, so I got some mockup done.

First picture here is the bike with the stock carrier (1977 CB750F) and it's hard to tell for sure, but you can see the front sprocket inboard of the rear sprocket.  (I just put an early output shaft in a set of empty cases (k3) that I have, easy mock up.)



And, it's pretty clear to see how the sprocket carriers differ.  Early model on left (grungy) late model on right (clean)



So, by installing the early carrier on the comstar rear wheel, things line up- here's a look down it with a chain on the sprockets.



So that's that.  Leaves you with one small problem though.



No matter what way you configure the spacers, you will need to fabricate a spacer as madmtnmotors was talking about.  With the early spacer installed on the early carrier, it needs to be ~14mm.  The late spacer is longer than the early spacer, but only by ~7mm so no matter which spacer you use, you need to cut an additional spacer to take up the slack lost by swapping carriers.

So the nuts and bolts for those that skim-  In order to put a early k engine in a late f/k frame, you need to do the following:

Swap rear sprocket carriers for the shorter, earlier version.

Cut a spacer to fit your particular combination of spacers.  If using the spacer from the early carrier, this is ~14mm.

Ride.

Easy Peasey, Japaneasy.

[andrewk]

Also, cycle-x sells a "3/8" offset front sprocket.  They sell them in 17 and 18 tooth, and it appears to me that it could be a bolt on solution.  The only downside is that the sprocket costs like 70 dollars.  I can turn a spacer out on a lathe for "signifigantly cheaper."  But for those willing to pay for a bolt on solution, it would be a good way to go.

Food for thought.

[seanbarney41]

Also, cycle-x sells a "3/8" offset front sprocket.  They sell them in 17 and 18 tooth, and it appears to me that it could be a bolt on solution.  The only downside is that the sprocket costs like 70 dollars.  I can turn a spacer out on a lathe for "signifigantly cheaper."  But for those willing to pay for a bolt on solution, it would be a good way to go.

Food for thought.

Does 3/8"=10mm?  ...just googled it 10mm=.393701" so yeah, close enough for a chain

[754]

 Make sure you use the correct inner "greasyspacer"

[andrewk]

Perfect name for that!  I am keeping them with their respective carriers, seems to line up like it should.  Using the late one with an early carrier would not be good though.

[Tugboat]

+1 - inner "greasyspacer" goes with the carrier.  Nice job everyone!!

THIS is why this forum is so fantastic.

[sethdhawkins]

I appreciate the legwork gentlemen.