25 mm Crank Pin (early C72?)

[arthur305]

G'day All, I got an interesting phonecall/question from a friend. He is looking for/hoping to obtain a 360deg crankshaft with 25 mm big end crankpin (later models have 26mm).
His reason - the big ends of his CR72 have not proven sufficiently reliable, and with the earlier crankshaft he can utilise Yamaha big end bearings, which are reputedly give a bit of longevity.
Yes he knows that the 360 deg crank shaft will have to be changed to run at 180deg - not sure that I understand it, but he is quite confident it can be done.

Can anyone supply related information, and (with a bit of luck) a possible source for one of the crankshafts with the smaller crankpins?.

Many thanks, Arthur

[G-Man]

Arthur

The cranks with 25mm pins are out there and I have a couple of early Dreams awaiting restoration. Bear in mind, though that the early 25mm pin was abandoned in favour of a 26mm pin when Honda moved to the sportier CB72. I'm guessing that the problem was flexing of the pin. They went as far as they could with the con-rod design they had - an expensive forging - and moved from 3mm rollers to 2.5mm rollers. I am a little sceptical that a CR72 with 25mm crank pins (unless it's just for parades) would be better than what he has now.

Resetting the 360 crank to 180 degress is reasonably straightforward. I built a jig for reassembly that will work for 360 or 180 pins.



I have a single NOS crank wheel with 25mm pin and have been using it to do a little research for repairing some of the worn cranks I have. Although this is a 25mm pin my intention is to fit a hardened sleeve to the 'normal' 26mm big end (with oilways modified in case of rotation) and then fit a Suzuki GS750 con-rod with a readily-available roller cage. The Suzuki rod is a little wide and will need to be thinned at the big end. The small end eye will need bushing, too. Honda CB360 rods are also a possibility but I found it harder to find a sleeve / roller cage that would work.



Ideally the sleeve should be a very tight interference fit and I am looking at high-temperature ceramic adhesives to offer a bit more security.



If I can make this work then I will be able to reclaim a pile of old crank parts I have building up.



Happy to discuss with your friend and would be interested to see some pics of his CR72.... :-)

G

[arthur305]

G'day G-Man, Many thanks for your reply and information. Firstly the bike is not in fact a "CR" Production Racer, but a "CB" raised as close to CR spec as the could manage.

He has lightened the crankshaft flywheels, and been using Honda Big End bearings. He also has discussed his engine with other CB72/77 specialists/racers here in Australia who have indicated that the switch to the 25 mm crank pins is a good idea, and does provide better reliability and longevity (they use Yamaha FZ bearings for the big end). One of the South Australian racers has a very successful CB77 (won most every classic race regularly) has over several years used these smaller pins and Yamaha bearing - with never a fail.
Turns out that my friend has probably been using an incorrect engine oil (he has used a 5w40 modern oil). The Engineer who will fo the engine conversion to the 29mm pins has said that these air cooled, roller big ended engines should use a single grade oil (Castrol R40 is I believe his recommendation). So time will tell.
As it turns out I had a 1960 C72 engine (ie an early one with 29 mm crank pins) in the shed (I was hoping to build it into a secopnd CB77 frame for my own road use) , but have supplied it to him to try getting his racer back onto the track.
So need to wait and watch. Cheers, Arthur

[G-Man]

G'day Arthur

It's always interesting to hear what people are doing with these bikes and Oz seems to be a bit of a centre of excellence.

I had some very interesting discussions with Jerry Kooistra who created some of the most successful of CB72-based racers. I think that his 350 and 500 bikes used a combination of CB72, CB350 and CB450 cranks. Not sure about the 250.

I have some of those Yamaha caged-roller bearing and was going to use them in a C72 build. I'm wondering if the 'difference' for the race guys is that the Yamaha cage has seperate rollers where the 'standard' Honda cage has them running in pairs. I know that the double roller design causes rubbing between the rollers and consequent overheating and failure at high speed.

There is a single roller cage that will fit a 26mm pin available from Rex Caunt here in the UK but it is a little narrower than the Honda big end and would nee a couple of thrust washers making.

Also, I think that Cappellini make a single roller cage for the CB72 which is similar to the original Honda 'racekit' design.

G

[arthur305]

Hi G-Man, Many thanks for your replies and information. My Friend has been in contact with most of the "more recognised" CB72 (250 and 350 versions) tuner/builders, and has been given the impression that the Yamaha big end roller bearing and the 25 mm crank pin is the way to go. However the crankpins in my 1960 C72 engine; whilst being 25 mm, are not good enough for a race bike. Not quite sure what is wrong, but that is for another time.
Do you have a pair of good C72 25 mm crank pins which he might be able to purchase - or do you know anywhere he might be able to purchase new ones?
BTW it is not actually a CR72, but a CB72 brought up to CR72 spec as far as was possible - it is still a SOHC, not twin OHC.
Thanks, Arthur

[G-Man]

Hi Arthur

Crank pins and rollers ultimately suffer from rolling-contact fatigue. Both pin and rod start to pit as the fatigued metal falls out. The highest stress point is just under the metal surface and even good looking pins rollers and rods can be just miles away from breaking up.







I have a couple of early cranks in a pair of Dreams that I am going to restore and there is no guarantee what condition they are in or even if they are 25mm pin types. The 'pin' is part of the inner crank web and the crank pin overlaps the main bearing so machining away the pin and replacing it is a very tricky operation. See my pics above.

It would be easier, in my opinion, to machine down a 26mm pin to near 25mm, reharden it and finish grind to 25mm if you could find the right machine shop.

I discovered that Yamaha XS650 pins are 26mm like the later CB engines but, as I said above, machining an interference fit into an incomplete circular bore could be quite tricky. You would also have to machine the big-end oil gallery

Here is the XS650 part.

https://www.mikesxs.net/parts/yamaha-xs ... er-pin-kit

Sorry I can't be much more help in that.

Let us know how he gets on.

G

Hi G-Man, Many thanks for your replies and information. My Friend has been in contact with most of the "more recognised" CB72 (250 and 350 versions) tuner/builders, and has been given the impression that the Yamaha big end roller bearing and the 25 mm crank pin is the way to go. However the crankpins in my 1960 C72 engine; whilst being 25 mm, are not good enough for a race bike. Not quite sure what is wrong, but that is for another time.
Do you have a pair of good C72 25 mm crank pins which he might be able to purchase - or do you know anywhere he might be able to purchase new ones?
BTW it is not actually a CR72, but a CB72 brought up to CR72 spec as far as was possible - it is still a SOHC, not twin OHC.
Thanks, Arthur