Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: billdn on May 21, 2022, 03:49:15 PM
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Barrels rebored to +.25 and pistons/rings supplied to suit.Bit of a fight to get barrels on but now pistons won't move up or down when turning crank. Moved fine before barrels went on, stripping back down to look but any ideas, please help me before I turn to drink !
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They should be tight Bill if the rebore was done to the correct tight tolerances. But, they should move. Did you put any assembly lube (Graphogen or similar) or oil down the bores?
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Another thought Bill, did whom ever do the rebore physically have the new pistons when the work was done?
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Check the ring gaps,
Do the pistons move in the bores without the rings?
Sent from my SM-A750FN using Tapatalk
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They should be tight Bill if the rebore was done to the correct tight tolerances. But, they should move. Did you put any assembly lube (Graphogen or similar) or oil down the bores?
Hi Julie,
Assembly oil galore on pistons and bores.
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Another thought Bill, did whom ever do the rebore physically have the new pistons when the work was done?
Julie, yes he had pistons , rings not fitted but then you wouldn't 😉
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Check the ring gaps,
Do the pistons move in the bores without the rings?
Sent from my SM-A750FN using Tapatalk
Took ages of gentle tapping to remove barrels, so tight and I mean no slide at all.
Chucked the towel in for today, but tomorrow will remove 1piston, measure bore with vernier as best I can and try piston with no rings, looks like bottom set on 1 are toast anyway.
Cursory measure made with steel rule and barrel diameter looks like 51.5, crude measure I know.
Keep you posted, thanks Julie and John
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Measure the ring end gap Bill whilst the piston/ rings are in the bores. This will tell you quite a bit.
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Measure the ring end gap Bill whilst the piston/ rings are in the bores. This will tell you quite a bit.
I suggest you attempt to slide a piston down the bore,
Use the piston to push the loose separate rings down the bore,it will keep the rings square in the bore,
Also,check the depth of the ring grooves, it is a slim chance they're not deep enough, causing the rings to protrude to much.
Sent from my SM-A750FN using Tapatalk
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That's tomorrow sorted then,
Rings and pistons from DS so should be OK but we've all been there before.
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Measure the ring end gap Bill whilst the piston/ rings are in the bores. This will tell you quite a bit.
Dumbo here 🤪 do you mean ring end gap or gap between ring and bore, the latter I guess use feeler gauge but the former I'm not sure 🤔
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https://youtu.be/ViEjQ7B1uM8
The above will give you a good understanding of what you are measuring with the end gap and how to do it Bill.
You don't measure piston/ bore clearance with a feeler gauge, you need a bore gauge to determine the bore measurement accurately. The difference between the skirt measurement of the piston and the bore is the piston / bore clearance. This should be about 1 thou inch maximum clearance. Unless you have measuring equipment with lots of decimal points, you won't get an accurate reading. I'll just have a look in the manual to see what the ring end gap should be as I can't remember.
Edit.... It was easier for me to take a photo.
[attachimg=1]
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I assume the new pistons were numbered from when the rings were gapped to each cylinder when it was bored out or have you done that part?
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Are you sure the rings were fitted the right way up? They are normally marked with a T which goes uppermost.
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Another possibility is a scraper side rail has twisted, been there
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Are you sure the rings were fitted the right way up? They are normally marked with a T which goes uppermost.
Oh yes made sure of that resorting to jewellers eyeglass to check profile of rings. Oddly not marked with a T but an R on top surface.⁹
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https://youtu.be/ViEjQ7B1uM8
The above will give you a good understanding of what you are measuring with the end gap and how to do it Bill.
You don't measure piston/ bore clearance with a feeler gauge, you need a bore gauge to determine the bore measurement accurately. The difference between the skirt measurement of the piston and the bore is the piston / bore clearance. This should be about 1 thou inch maximum clearance. Unless you have measuring equipment with lots of decimal points, you won't get an accurate reading. I'll just have a look in the manual to see what the ring end gap should be as I can't remember.
Edit.... It was easier for me to take a photo.
(Attachment Link)
Useful clip Julie, explained it well. The photo from manual I gave a photocopy of to engineer shop. Hopefully he read it 🙏 😬
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Another possibility is a scraper side rail has twisted, been there
Could well be as it was roughly around that point that things started to get difficult when fitting barrels. But all 4 ,although they are very fine and came in 3 parts. Something else to rule out.
Cheers Bryan
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The R is a different make, all ltters/numbers face upwards
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No. 3 piston out as oil scraper ring bent, 🤨 good shout Bryan ,sadly. Also left score in barrel, back to engineering shop tomorrow to see if salvageable 🤞.
Ring end gaps on first and second ring no greater than 0.25 ,as 0.20 went through but 0.20 +0.05 wouldn't, oil scraper gap no more than 0.40.
BTW piston without rings on slid perfectly through all bores.
Stay tuned !
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I assume the new pistons were numbered from when the rings were gapped to each cylinder when it was bored out or have you done that part?
Not quite sure what you mean Ted?
It was a full rebore to +.25 and new pistons/rings to suit , would they need matching to each bore separately?
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I assume the new pistons were numbered from when the rings were gapped to each cylinder when it was bored out or have you done that part?
Not quite sure what you mean Ted?
It was a full rebore to +.25 and new pistons/rings to suit , would they need matching to each bore separately?
When Graham bored my block it was sent back to me with the rings fitted & the piston crowns marked with the cylinder bore with an ink marker. If you gapped your own rings then you would presumably match them as you went along. I'm assuming that in reality each piston will have a tolerance as will each bore so there will be clearance variation for each ring in each bore before you gap them imho. I'm not an engineer - I have found through experience with car engines that some rebores are not as exact as they should be. That's why I rate Grahams skill in this area. Sadly the place I used for cars as long since closed - Wylde Engineering in Derby were old school. Do you think the ring was bent when fitting the pistons,? I found that really difficult as you can't see or really feel what's happening. All my previous piston fitting on cars has been from the top
Hopefully the bore marks are not too deep and can be honed out.
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In 40 odd, some say very odd, years i have never had to gap a japanese piston ring.
I have had a scraper rail twist building a 1200 wing so it can happen to the best of us, dont feel bad about it, as long as the scratch is not too deep it will run, it al depends on what you realisticly want the bike to do
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In 40 odd, some say very odd, years i have never had to gap a japanese piston ring.
I have had a scraper rail twist building a 1200 wing so it can happen to the best of us, dont feel bad about it, as long as the scratch is not too deep it will run, it al depends on what you realisticly want the bike to do
Interesting to read about never having to gap Japanese rings Bryanj
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Their QC seems to be good, to be honest never had to gap Hepolite either, as long as you ordered the correct size in the first place,
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I ‘gapped’ the ones I bought from Japan and they were all spot on. It was an annoying but necessary waste of time.
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Footnote to above gaps on all rings were spot on, so can't blame rebore.
On plus front the engineer reckons where the scratch is below rings even at lowest point so rebore not necessary 😉 . To be on safe side thought I'd order new ring kits for all pistons. Weirdly +.25 ring kit (genuine Honda btw) £29.13 whereas after market replacement full piston kit inc rings etcetera just 90pence more. Go figure 🤔. So ordered full set again .
BTW anyone want 4 unused +.25 pistons sans rings? Thanks for your comments,advice and support all.
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Use the original set of pistons with the new rings you are buying Bill. The rebore would have been done to those pistons, not the replacement pistons you're buying.
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Use the original set of pistons with the new rings you are buying Bill. The rebore would have been done to those pistons, not the replacement pistons you're buying.
Good call Julie, hadn't thought of that .
Cheers
Bill