CR750 Replicas

[Phil]

Hi - I am putting together a CR 750 Replica and would welcome advice, photos etc of the genuine article. I have two engines - a K7 and a K6. I believe a K7 is an unpainted F2 and will have bigger valves and combustion chamber (?). Similarly it has a closer ratio gearbox. I intend to use a special 836cc Dynoman piston kit, race cam (Megacycle or Dynoman), s/steel valves with titanium retainers, stronger springs and Keihin CR 31mm carbs. I want to use standard rods. I need someone to port the head and fix the new valves and guides - anyone know anybody? The K6 engine will be tuned to rev higher and will use a Nova c/r gearbox and Carillo rods, etc. Any help/advice most welcome!! Phil (philipclark2@mac.com).

[Voxonda]

Hey there Phil,

First the K7 is not a unpainted F2, I personally think the K7 is the better of the two. The advantage of the F2 is that it produces more power, but at a cost. The head of a F2 is crap, due to the sharper cam and the bigger valve the valveguides wear out very rapid, hence 'smoke' and oil consumption. Have build a Japauto Endurance replica with a K6 engine and am in the process of building a 850cc Turbo based on a F1 engine. Look out if you're using a sharp cam and heavy springs, the construction of the 'camtowers' cannot take to much stress. There are lying a lot of blown engines around.

Cheers, Rob

[Phil]

Hi and thanks for the response. I have obtained parts books for F2 and K7 (thanks to this great site!) and you are correct: the valves on the K7 are the same as K6 and different to F2. I also note that there are slight differences in the gearbox. The K7 and the F2 are identical, according to part numbers, but different to the K6. Is this the 'closer ratio gearbox'? I want to run a standard gearbox on this engine so this is a bonus - or am I missing something?

Phil

[SamCR750]

Hi Phil, check out my bike in this thread. http://forums.sohc4.net/index.php?topic=25400.0

Also check this thread in same forum.  http://forums.sohc4.net/index.php?topic=27902.0

PM if you need any further help.

Sam.

[Phil]

Wow! That is a lovely bike! The thread is interesting. The original CRs are very rare birds indeed. The only one I have seen in the flesh is in the old Honda museum at Suzuka. Sadly they would not let you take pictures (they do now). Is that the standard rear hub on your bike or one of those expensive magnesium jobs? What are the footrests - Disco Volante? Did you beef-up the cam holders? I have 101 questions ...
cheers,

Phil

[SteveD CB500K0]

101 questions is good.

Catch Sam at the Woodbridge Sprint in May (10th I think).

I'm supposed to be riding in the Thundersprint Cavalcade that weekend (on the Rickman) so will miss him again....

[SamCR750]

Wow! That is a lovely bike! The thread is interesting. The original CRs are very rare birds indeed. The only one I have seen in the flesh is in the old Honda museum at Suzuka. Sadly they would not let you take pictures (they do now). Is that the standard rear hub on your bike or one of those expensive magnesium jobs? What are the footrests - Disco Volante? Did you beef-up the cam holders? I have 101 questions ...
cheers,

Phil

The rear hub is stock Phil but has been converted to twin leading shoe.
The footrest are home made to CR spec.
The cam cover has welded on bosses that line up with the cam holders bolts. Bolts are threaded through the bosses untill they touch the cam holder bolts, then they are locked into position with lock nuts.
See it in the flesh at a track near you on the 10th May.

Three down, 98 to go.

Sam.

[SamCR750]

101 questions is good.

Catch Sam at the Woodbridge Sprint in May (10th I think).

I'm supposed to be riding in the Thundersprint Cavalcade that weekend (on the Rickman) so will miss him again....

Is the Thundersprint not on the 11th Steve. We will be there if all goes to plan on the Saturday.

If we can get it on time, the bike will have a fully covered front end in the shape of a Georg Brown/Nero dustbin.

Sam.

[SteveD CB500K0]

Yes Sam, but the "scrutineering" for the cavalcade starts at 0700 on Sunday. We'll be in a hotel...

[Phil]

Thanks again Sam. I will be there if at all possible. Q96 - do you use 31mm or 29mm Keihin CRs? I am told to use 29s as my first engine will use standard con rods and therefore won't be able to rev high enough to enable the 31s to work. This is a "parade" engine - I am planning a Wiseco 836cc kit, Dynoman/ Megacycle cam, stronger valve springs and some mild headwork and the CRs. I am tempted to try a standard'ish F2 head? I have plans for engine 2 which I want to keep 750 with a light crank, Carrillos, Nova box etc. Q95 ...

Cheers
Phil

[Phil]

Woodbridge Sprint? Thundersprint? Northwich? Am I missing something? Where will you be on May 10/11th Sam?
Phil
 

[SamCR750]

Thanks again Sam. I will be there if at all possible. Q96 - do you use 31mm or 29mm Keihin CRs? I am told to use 29s as my first engine will use standard con rods and therefore won't be able to rev high enough to enable the 31s to work. This is a "parade" engine - I am planning a Wiseco 836cc kit, Dynoman/ Megacycle cam, stronger valve springs and some mild headwork and the CRs. I am tempted to try a standard'ish F2 head? I have plans for engine 2 which I want to keep 750 with a light crank, Carrillos, Nova box etc. Q95 ...

Cheers
Phil

Phil, the 31s work great with stock rods. If you go with Wiseco 836 forged pistons, the Megacycle 125-65 camshaft, and a good port job with decent valves, the motor will make more power than the factory CRs without putting undue stress on the rods as the rev limit will remain stock at 8,500rpm. what you will have is a motor with enough torque to make you think twice about cracking it wide open in the lower gears.

Sam.

[SamCR750]

Woodbridge Sprint? Thundersprint? Northwich? Am I missing something? Where will you be on May 10/11th Sam?
Phil
 

The Sprint meeting at Woodbridge is on the 10th and 11th May. We will run on the Saturday and if we have no problems and the bike achieves what we expect from it, we will leave it at that and head home for the Thundersprint at Northwich on the Sunday. If we have any problems we will stay at Woodbridge untill we get the job done.

Sam.

[Phil]

Q92 and Q93 (don't want to overload Sam!) - I am hoping to race/ parade this replica in CRMC events and the rules are strict and confusing! I need to get alloy rims put on the standard hubs. Anyone know A) the max rim size that is allowed (and that the swinging arm can accommodate); B) the best tyre size/ make? I am also using the standard stainless discs because I can't find steel replacements (anyone know a supplier?). The s/s discs weigh a ton and say on them that the minimum thickness is a wopping 6mm! My s/s NSR discs say maximum thickness is 3.5mm. Does anyone have experience of machining discs? Does anyone know where I can get the discs drilled? Thanks!

Phil

[SamCR750]

Can't help with either there Phil, we stuck with stock rim size (WM2/WM3) and stock brake discs.

Sam.