Hi
I have assemlbed the bits to build a 350cc CB77 and read an old UK road test of the Read-Titan bike in which they mention "re sphering" the head as part of the cost.
As the 350's were more popular in the States, can anyone who ran one tell me if they had to do this to blend in the edge of the squish band on a CB77 head to match the enlarged bores ?
If you don't do anything does this lead to a hot spot and detonation ?
The engine is intended for road use and will not be tuned in any other way.
thanks in advance,
Phil
Should a CB77 head be machined for a 350cc Kit ?Phil
What pistons are you using? If you're using CB350 pistons they will need re-machining (valve pockets and squish area. If you are using hepolite 'big bore' pistons (copies of the forge true) or forge true, you should be OK unless the head has been skimmed. I'm building my own big bore kit using CB750 liners and either CB350 or forge true pistons (I have both). You might want to build up the engine 'dry' and use 'plastigauge' or solder to test the clearance G
Last edited by G-Man on Fri Jun 10, 2011 1:34 am, edited 1 time in total.
'60 C77 '60 C72 '62 C72 Dream '63 CL72
'61 CB72 '64 CB77 '65 CB160 '66 Matchless 350 '67 CL77 '67 S90 '77 CB400F Some pistons have been machined to suit a stock head shape. Back in the day, we used forgeture/Webco or CB350 pistons and they had a different squish angle.
Cb750 pistons are almost flat top and are not suitable unless you have 12.5:1 pistons in 64mm size. Easiest to modify are CB350 pistons but 64/65mm pistons are now available from Bore-tech and Les Barker. Head gaskets are available from both or from Cometic. Thanks G-Man and Teazer,
I've got CB350 pistons and a copper 64mm head gasket from Classic Gaskets here in the UK. The pistons seem to have a lower gudgeon pin to crown height than the CB77 pistons I have but will dry build to check clearances. What I was more worried about after reading the article was that the edge of the CB77 head might be an issue as it sits inside the new bore, but I've since found a 1971 MotorCycle Sport Quarterly Technical magazine that includes a detailed article about fitting the Webco kit and there is no mention in that of having to modify the head so guess I'm worrying unecessarily. Similarly I take it there is no issue with the original CB72/77s having offset gudegeon pins and the CB350 ( I assume ) not. cheers Phil
Phil,
You must have read the same article as I did many years ago. I fitted a Reid Titan kit to my CB77 and I did relieve the head as per the article as it seemed to make sense. I don't know that it made any difference but I have had no head-gasket problems in the 30 years and thousands of road miles since. I made an aluminium head gasket, softened it by heat treating in our oven and it works good. A very handy tip before you do that is to take out the aluminium oven trays first as they get very soft as well and the wife is not happy. Oh well! Leon.
CB350/ big boreHello Phil
Just a thought, if the gudgeon pin to deck height is shorter by much, perhaps the cyl. block will need to be skimmed and not the head if you dont want to low a comp. ratio.? Gordon SAFOJ Overall piston height and distance from pin to top of the crown are similar on both pistons, but CB350 pistons have an angled squish that's not compatible with the CB77 head shape.
I usually machine CB350 piston crowns to look more like a CB77 with either a flat or angled squish area (your choice) and then the valve pockets need to be machined and the head has to be opened up. Do that and you basically have a slightly higher compression 350cc CB77. You may need to machine to top of the barrels to get the correct deck height. The way they did it back in the day IIRC was to machine a shallow squish band into the piston and leave it at that. The piston can probably be machined to obviate the need to machine the head. Compression would be lower though. Or you can take another couple of mm off the head and change the squish angle completely and machine the pistons to match. That would give you a CR suitable for race gas or methanol and not much use on the street. CB350 and 77 share the same thick heavy rings, so revs are a bit limited before the rings flutter but that's not a problem on the street. One approach would be to get the new sleeves fitted and bored and honed and build the motor without rings and look at the crown protruding from the block and work out the line of least resistance from there.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
