Hap Jones Pistons for CB72

teazer · Post by **teazer** » Sun Aug 22, 2010 10:02 am

Welcome Colin. I rebuilt my old CB72 350 race bike when I was home in Melbourne a few weeks ago. Wish I was going to PI to race it.

What is your race number and what mods have you made to the bike?

ColC · Post by **ColC** » Sun Aug 22, 2010 7:47 pm

Hi Teazer
The 350 has a nova 5 speed, asso high compression pistons, bigger valves and have just fitted 32mm amal carbs replica CR77 alloy tank. The 250 is very similar but runs original honda sport pistons 1mm oversize and factory 5 speed box. The 500 is one of Jerry Kooistra's creations and I'm pretty sure the forum is familiar with his modifications. My son will be riding the bikes at the nationals as my balls arent as big as they used to be, his number is 46. he was the period three 250cc champ on the cb72 in 2008 and 2009. I will post some photos soon but will have to get one of my kids to do it for me.
Cheers Col

teazer · Post by **teazer** » Sun Aug 22, 2010 10:45 pm

Ah so. That all makes sense. I have some pictures my son took of the bikes at the island or Broadford. I don't remember which.

Very Nice.

Original Honda 5 speeds are rather rare and somewhat fragile when power gets over a certain level, so it's good to use it in the 250. When I built my 350 all those years ago, nothing much was available. I used to import cams form Norris (that dates it) and for pistons, the only available option were modified CB350 so that's what we used. It took a few sets to work out the optimal crown shape and to work out how much to take off the barrels and head to make it all work.

I should sell that bike, now it's all back together. Any ideas on the best place to advertise it in Australia? I have a NOVA 5 speed which could go in. http://pinkpossum.com/CB72/index.htm for details. That needs to be cleaned up a bit but tells some of the story.

Of course it's not a Kooistra quality build - unfortunately. I used to think my bikes were excellent until I first clapped eyes on the first 500 (450) that Jerry built and then I realized they weren't quite as good as I'd thought.

Good luck at the Island. I used to love racing there, though I managed to eat a few motors there over the years. It can be hard on motors

Spargett · Post by **Spargett** » Mon Aug 23, 2010 1:51 am

Cool bike Richard, love the shots on the track.

ColC · Post by **ColC** » Mon Aug 23, 2010 2:36 am

Thanks Teazer
I have found bikesales.com.au to be pretty successful over the last year or so cost $50 add stays online until bike is sold. However you have to update it each month, but they send you a reminder.
Yeah the asso pistons are pretty good and tend to go straight in without to much mucking around.
Why dont you race rhe CB77 in the states?

Snakeoil · Post by **Snakeoil** » Mon Aug 23, 2010 9:19 am

Very nice job on that road racer. I'm curious about the combination of parts. When you say you used CB350 jugs, I'm curiou why you kept the CB77 head. Does that head flow better than the 350 head or is it a class rule limitation?
regards,
Rob

teazer · Post by **teazer** » Mon Aug 23, 2010 2:50 pm

Thanks Col. I'll try that.

Rob (Snakeoil) I use CB72/77 barrels and remove teh sleeves. I shorten suitable sleeves from CB750 or similar and bore the barrel muff to take new sleeves. Sleeves are then bored out to suit the pistons.

In my case I don't like lumpy pistons and Barber Racing proved that they stop the motor from revving and get in the way of making power. Back in the day I just didn't like big lumps and long thin combustion chambers so I tried a few different crown and combustion chamber shapes.

What we ended up with is an angled squishband around teh edge of the crown and a different shaped lump on top. Barrels were shortened significantly and the heads are machined to match the squish angle and then valves are recessed slightly and unshrouded.

The head shape is neither CB350 nor CB72, but not as radical as JK's or as the CB160 240 motor which has welded up bath tub (polyspheric) chambers. What I needed was high enough CR with a compact combustion chamber with good quench and turbulence.

Also used larger valves for a while but with that long 54mm stroke and small ports, the revs were too high and it ate valve gear. My compromise was stock but lightened valves and an 11,500 red line and change piston rings regularly. Thick rings can't stand the acceleration over about 10K.

If I built another motor I'd get sets of pistons from JE made to my spec and I'd remove the cast skull and weld it and the port floors and start again.

We have been building and racing CB160s and TZ Yamahas and now I'm neck deep in GT750 reed valve projects. The more I learn, the more projects I seem to be involved with and I've maxed out the number of hours in the day. Bikes are "just" a hobby......