Hap Jones Pistons for CB72

davebern · Post by **davebern** » Wed Jun 02, 2010 1:06 am

I am assembling a CB72 motor with Hap Jones S12 pistons. Measuring the compression volume gives me 20cc (that's on an assembled motor at TDC), which works out at 7.8:1 compression. Is this right?
I found this quote on a CB175 racer site:
"Stay away from the common "Hap Jones" 3mm (55mm) overbore pistons...they are crap and produce about 8:1 compression. " which makes me worry about the pistons I have fitted. Anyone out there have any experience in this area?

Dave in Australia

jensen · Post by **jensen** » Wed Jun 02, 2010 1:13 am

Hi Dave,

Welcome to the forum, where are you located in Australia ?
From what year is the CB72 ?

I also installed a set of Hap Jones in a CB72 once, and I cannot remember something bad about them.
It has indeed a low compression, but are lighter then standard Honda Pistons.

I changed them after 10 K km, and they where worn, but that's kind of normal, also for Honda pistons.

Jensen

davebern · Post by **davebern** » Thu Jun 03, 2010 5:40 pm

Hi Jensen,

I am from Ryde, in Sydney, Australia.
The numbers show my bike as a 1961 model, but there are a few things that are not right, such as the speedo has 2 screws holding it in instead of the clip the 61 models are supposed to have.
It was given to me in 1985 in very bad condition, so it's history is unknown.

Here are before and after pictures. I hope you can see them.

http://yfrog.com/3mshondabefore2j
http://yfrog.com/b8cb72l2j

Dave.

LOUD MOUSE · Post by **LOUD MOUSE** » Thu Jun 03, 2010 5:58 pm

May we have the frame and engine numbers.
Sure looks like a 1964 or later other than the rear axle. .................lm

davebern wrote:Hi Jensen,

I am from Ryde, in Sydney, Australia.
The numbers show my bike as a 1961 model, but there are a few things that are not right, such as the speedo has 2 screws holding it in instead of the clip the 61 models are supposed to have.
It was given to me in 1985 in very bad condition, so it's history is unknown.

Here are before and after pictures. I hope you can see them.

http://yfrog.com/3mshondabefore2j
http://yfrog.com/b8cb72l2j

Dave.

teazer · Post by **teazer** » Thu Jun 03, 2010 6:17 pm

I don't know nearly as much as LM or some of the other guys on this site, but the wheels with clamp type brake arms are later than 62 for sure.

It sure is purdy though. Nice job.

If you want to increase compression and already have the pistons, you can machine around the "squish band" of the pistons and then machine the block to get zero deck height. That will give the pistons a taller crown. And then maybe take some off the head to increase compression.

leave at least 2.5mm of top ring land or else the land may collapse.

OR get a 305 barrels or a set of 350 barrels. That reminds me I have a set of 8 hole barrels already fitted with new 350cc sleeves with top and bottom faces machined. They are in Melbourne too. I'd have to check them when I get back there next week to see what size the liners were machined to.

You have options out the wazzoo.

jensen · Post by **jensen** » Fri Jun 04, 2010 12:16 am

Hi,

I'm afraid that this isn't a 61, and possibly not a (late) 62 either, but it is a nice bike.
The screws where used in 61 and 62 bikes, only the very early 61's have the clip that holds the speedo.

Jensen

davebern · Post by **davebern** » Fri Jun 04, 2010 2:51 am

The numbers are a bit mangled, but the frame number looks like 100020, and the engine number is 100023 (from memory, the bike is at home). I do remember reading somewhere that some '64 models started with 1 before they went to the longer numbers which all started with 1, so it probably is a '64. A few people have looked at it and said it'a a '61, so I didn't argue, even though I had my doubts.

With regards to machining the head and barrels, what do you guys do about getting the cam timing correct again? It looks very difficult to index the cam sprocket.....