Subject - CB72/77 Piston - Cylinder Clearance
Hi
I've looked in the original Honda manual and done a quick search but can't find what the minimum
clearance between the piston skirt and the cylinder is ? The manual mentions a service limit i.e.
the maximum clearance, and comparing the dimensions of the std piston and cylinder suggests
anywhere between 0.00mm and 0.01mm.
Reason I ask is that I've picked up a couple of sets of CB72 barrels that claim to have been
bored out but using a bore gauge they don't seem to line up with the standard oversizes.
Realise it could be me not using the gauges accurately but before I send them off to be
rebored or go looking for matching pistons it would be good to know what clearances people
are using.
Given the reputation for nipping up and the fact that the skirts are so long ( compared with modern
engines ) do most people allow a bit more clearance when reboring ?
thanks in advance,
Phil
( in the UK where you can have all 4 seasons in 1 day ! )
Minimum clearance piston skirt to cylinder ?
Hi Phil,
I use 0.01 (+0.01, -0.00) mm, that means somewhere between 0.01 and 0.02 mm and that is what honda specifications are. The only way to achieve this is doing it right : First fit the liners in the cilinders (if you refitted new liners) If you have more pistons to choose from, choose the pistons with exactly the same diameter (sometimes pistons differ more than 0.02 mm in the same set, and I have seen more difference in one set. The tolarance is bigger than the the claerence to obchieve, so do it carefully). Make sure that while measuring the temperature of the pistons and cilinder is 18 to 21 degrees !!! After choosing the pistons make sure that they have exactly the same weight, if not make them having the same weight, mark the pistons as left and right. Let the holes be bored and honed (use a two stage hone proces) according to the piston diameter, and make sure that you fit the right piston in the right hole. Make sure that the piston rings fit, if the gap is smaller then the specifications, make the piston rings to the specifications, use a feeler gauge to verifey. Brake in the pistons with a mineral or half synthetic oil. Make sure the mixture is not to lean while breaking in, it´s better to let the mixture a little richer by set the needles one clic higer then normal. After braking in you can put the needles back in the right position. This will set you back around 90 euro per hole to get it right. I never had a seizure with any forestroke I rebuild. If you think that a bigger clearance is better, think twice, it WILL cause piston slap and apart from exsessive noise it will speed up wear to cilinder bore and piston (and small end and big end) Jensen assembly of Japanese motorcycles requires great peace of mind (Pirsig)
Phil,
If you have the luxery of having more pistons then two, it's wurth it. If you only have two pistons make sure that you bore the hole to the piston diameter for each piston. What realy helps is knowing someone who can do it for you, including measuring. If you have the posibility of turing the pistons out (remove a little material from the inside) to make the pistons exactly the same weight, you should do that too. I know that it sounds a little OCD (thanks steve), and if you think it's not worth it, that's ok. It's an advice, not a rule. But if you talk about 1 or 2 hundreds of a mm you have to think in a different way. Maybe it's fun to measure (and calculate) the difference in piston diameter between -20 degrees (your fridge) and 100 degrees (cooking water) and you 'll get the feeling. Make sure that you know WHERE to measure the diameter. Pistons are NOT just round pieces of aluminium with two holes in it. In general the piston has a smaller top diameter. Jensen assembly of Japanese motorcycles requires great peace of mind (Pirsig)
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