Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: Eldrick on June 10, 2024, 06:06:42 PM
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During my gasket chage I noticed my pistons are black, see this image.
[attachimg=1]
Do I need to replace these pistons?
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They are running rich as you will know from the colour - if you had good compression & the bores look good with no step at the top I would just clean them.
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I agree with Ted. I personally would remove the pistons from the rods to a) clean the pistons of the carbon and b) to make life a lot easier when removing the base gasket and cleaning up the joint face. Get some clean rags down those openings before you do anything else, there's a lot of dirt and grit around there.
You'll need four new piston circlips when replacing the pistons on the rods (you only need to extract the circlip from once side when removing them).
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No need to replace it is just carbon build up. Giving them a clean is enough, as it has been mentioned before.
Are you running your engine at a low RPM usually? Something like bellow 3000?
I have been told that those deposits form when the combustion chamber temperature is slightly too low, that is when the engine is run at low rpm, whereas at higher rpm the deposits burn off, keeping the piston heads clean.
Something something italian tuneup
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My pistons are factory stamped with a number and instead of 1-2-3-4 they are 2-3-4-2. Since the bike was running just last week, should I be bothered that a 'PO' has used non-sequential pistons?
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If it ain’t broken don’t fix it.
I’d be interested in what the numbers mean but it is not necessarily a piston order.
Maybe it is the stamp for the mold reference, so that in case of a manufacturing defect you can easily spot the faulty mold. You can see that on many cast products.
Not saying that is the explanation but it could be one.
Either way if your bike ran fine don’t bother. Feel free to check their specs are within tolerances but yeah other than that refer to the first sentence of this message hehe.
Motorcycle maintenance can be tricky and stress inducing and we always assume the worst very quickly, but if there is an issue the bike will make it loud and clear.
Trust your senses and your smarts and keep cool!
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Can't believe they were numbered by Honda in assembly, most likely a casting batch number.
Although my experience is mainly automotive never seen pistons numbered during factory production.
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Here they are, I presumed the number was a correlation to the piston barrel number.
[attachimg=1]
Whilst I am talking about those, how do I know if the rings need replaced?
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The rings are here to separate the combustion chamber and the oil lubricating the piston. Busted rings would result in oil burning in the chamber, which is signalled by a blue smoke at the exhaust and the oil level dropping over time. Other result is a loss in power as the compression goes into the crank (seal of the rings gone).
As the lubrification is compromised the pistons can end up grinding against the piston sleeves, so vertical grind lines on the sleeve can indicate poor lubrification and so busted rings.
Also if your rings are broken well then clearly they need to be changed.
If you haven’t noticed anything then no need to undertake changing your rings. Again if you want to check that all is well you can refer to the shop manual where you’ll find the ring’s tolerances and check that out
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Those are not bore numbers but probably mold numbers to identify any production problems
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Did you check the compressions before taking the top end off. Looks like some surface rust on some of the rings.
I would be checking the pistons for scratches/ wear on the thrust side plus looking at the bores for any marks from standing.
I'm no fan of fitting new rings on old pistons & into existing bores.
Likewise I would not go for a rebore without checking the crank bearings.
If it was running okay aside from the oil leak you might be okay fitting them back as they are.
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I don't have a guage to check compression.