Alternative Pistons - CB750 ?

Phil-UK · Post by **Phil-UK** » Fri Apr 23, 2010 3:03 am

Hi

A question from a friend - he has picked up a set of CB77 barrels at +1.00mm that could
do with a rebore, and is wondering if he could use CB750 pistons ( std bore = 61mm ).
Anyone ever done this or heard of it .. obviously issues like gudgeon pin size and height of
crown come into it but Honda do seem to keep to certain sizes.

He is looking for the cheapest and least complicated way to sort it out.

An alternative may be the Hap Jones pistons .. I know they did larger than standard O/S for
the CB72s so is the same true for the CB77s ?

thanks in advance

Phil

davomoto · Post by **davomoto** » Fri Apr 23, 2010 9:08 am

Phil, I think I remeber hearing that the CB750 pistins will work with some modificatio, but they are flat top pistons resulting in a lower compression ratio. The 3mm over kits come up on ebay from time to time. In his position, I would look for boreable sleeves. They are quite easliy replaced, put cylinder in the oven at 250 degress until they literally fall out. Put the replacement liners in the freezer for a few hours, cylinder back in oven for 20 minutes, and drop the replacement liners in. Or, find a compete boreable cylinder, or Tim at classic honda sells standard size sleeves, cost is not much more than boring.

davomoto

teazer · Post by **teazer** » Fri Apr 23, 2010 9:36 am

CB750 pistons are fine for a 7:1 cr or something like that. Even with modification, the CR is still too low.

Davomoto has covered the options pretty comprehensively

Phil-UK · Post by **Phil-UK** » Fri Apr 23, 2010 2:19 pm

Thanks Guys,

Always worth posing the questiion I suppose.
The new liners are probably not an option certainly by the time you add shipping to the UK
and Import Duty and then VAT.
I'll tell him to keep an eye on US Ebay for the 3mm O/S pistons.

I went for the full 350cc conversion and got some std CB350 pistons on Ebay and managed
to have a friend bring them back in their luggage. Not built the engine yet but should go well.

cheers

Phil

G-Man · Post by **G-Man** » Fri Apr 23, 2010 2:53 pm

Phil

I know a guy in the UK who has barrels and liners, both standard and big-bore.

PM me if you need something. I also travel to the US quite often if you need some pistons picking up.

G

teazer · Post by **teazer** » Fri Apr 23, 2010 6:25 pm

Phil,

CB350 pistons are not bad but need some machining to work. And the liners are really too thin at 64mm. You might want to grab a set of Cb350 or CB750 liners and modify them as required.

Phil-UK · Post by **Phil-UK** » Sun Apr 25, 2010 10:39 am

Hi

G-Man thanks for the offer, and I'll pass on the bit about new liners to my friend.

Teazer - I had new 350cc liners fitted to bored out CB77 barrels so I'm happy things will be
OK, someone I know was running the same set up with CB350 pistons until a disagreement
with a lorry but he's almost finished the rebuild now.

thanks again

Phil