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Discussion Starter · #1 ·
I have some vertical scoring on the R.H. cylinder wall on a CB450 K2 engine that can be seen by peering through the plug hole. Compression is down at 135psi (hot) on both sides, though a teaspoon of oil gets this up to 185psi. My thought is to take the barrels off to take a look what's going on. My gut feeling is it will need a rebore and pistons. If this is the case, does anyone know what the quality is like of the David Silver 'replica' aftermarket items? OEM works out fairly expensive.
 

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I and friends have used a lot of pistons from that eBay seller (cruzinimage_co) and they are very good quality. Never had any problems with them. Also, they are from Japan, not China. Probably from the same foundry that made the original pistons for Honda.
TOOLS
 

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Discussion Starter · #5 ·
Both of those sellers look to have nicely machined pistons, though that particular Japanese seller doesn't post to the UK. I wonder if the David Silver pistons are Japanese? I need to send them an email to ask. There are also variations in the pictures on the David Silver UK site - the standard pistons have the crowns machined whilst the oversizes are 'as cast' and seem to show different oil control rings.
 

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I purchased a pair of CB350 pistons from the Scrambler Cycle referenced above. I haven't gotten to run the engine yet, but I had my machinist check out the pistons before I installed them and he put his stamp of approval on them. IIRC, they were like $100 for the pair too which didn't break my heart!
 

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Remember, there are differing clearance requirements for forged and cast pistons....
Make sure you and your machinist follow the piston manufacturer's specs, which may or may not necessarily match the original Honda specs...
 

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You get what you pay for. It would be nice if the ebay seller posted better pictures. The scrambler-cycle pictures aren't much better but at least they show a side and bottom view. The S-C pistons are sand cast. There is nothing wrong with that, but they look kind of sloppy on the underside. That may or may not be a problem. If I used these, I would try to source a set of OEM circlips for the wrist pins. I don't like the looks of the style that are included with the ebay set.
Like what Sprint said, clearances are different between cast and forged pistons. Stock pistons are cast. The WISECO pistons are forged and the compression ratio is higher. Forged pistons are tougher than cast.Typically cast pistons run closer tolerances than forged. The looser clearances of forged pistons have less drag allowing the engine to rev quicker, but they are noisier.
If you see a set of ART pistons, those were marketed as early after market pistons. ART was the piston supplier to Honda back in the day and they match the OEM spec.
Don't bore the engine unless you do the valves too and make sure the rod small end clearances are within spec. You should wind up with a great running bike. Keep us posted.
 

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Discussion Starter · #11 ·
I contacted David Silver about their pistons. The reply was they've checked the stocks and they're the machined crown type - not the 'as cast' types in some of the pictures. The packaging looks like they're the same product as the Scrambler Cycles. I'm about to remove the head and barrels today to do an inspection. The bike had been standing since 1983 and the best I got with repeat applications of Marvel Mystery Oil and ATF/acetone was 135lb psi on both sides, and it smokes badly and oils the plugs. I was hoping for better, as the bike is in pretty tidy, rust-free condition overall. Wiseco pistons would be a nice choice, but would run me to over $506 (£385). I'll see how it goes today.
 

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Discussion Starter · #12 ·
I received the pistons from David Silver. There are some differences to the originals - the inside is not as tidy and the crown is machined differently and is very slightly higher. The oil control ring on the original is the slotted type but the replacements use a three-piece assembly with an expander ring. Not too bad for the money, but the big letdown is the gudgeon pin wall thickness and extra weight. The original weighs 53g but the pattern part is 60g. All-in the original piston assembly (with rings and pin) weight 302g but the new set is 310g.

Some weight allowance needs to be made for the replacement piston being 0.025mm oversize - about a gram or so. This still makes the replacement set around 7g over weight. Is this likely to upset the balance?

Here are a couple of (slightly fuzzy) pics;

http://www.hondatwins.net/forums/images/attach/jpg.gif
http://www.hondatwins.net/forums/images/attach/jpg.gif
 

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You're worried about a few grams difference on an engine that is already a moderately vibrating 180° twin to begin with? I suppose you could trim the skirts a little to get closer to the OEM weight, but I don't think you're going to notice much either way
 

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Discussion Starter · #14 ·
I'm thinking the pins would be the place to remove weight, though they may be too hard to cut in the lathe even with ceramic/carbide tools and low speed. Best left, I think, and get it back together with what I have. It's a poor situation where it would be easy to manufacture the pin with the correct wall thickness to begin with.
 

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Those pistons don't look all that bad - the typical "ART" pistons we see look much more crude and funky.
They should be fine, I'd use them as-is.
 

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Discussion Starter · #16 ·
Thanks for the replies on this - I'm taking the pistons and barrels to the machine shop to get them bored tomorrow and will put the engine together with the piston assemblies as-supplied.
 

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Look at it from a percentage wise and you are well within tolerances. Far more important is the difference between assemblies, ie is one piston close to the same weight as the other piston, that's what race engine builders do.

I make an attachment for a tool that oscillates at 11 to 12 thousand motions per minute. There are four pieces involved and my target weight is 60 grams. My range is 58 to 62 grams. Sounds like a lot until you do the percentage and in field tests it makes no difference to longevity or functionality.
 

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I'll be doing an engine rebuild shortly Mickyluv and very much the same as you I would think, interesting and helpful read so far so I'm looking forward to reading any further installments if you would? Thank you.
 

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the crank balancing weights calculated are essentially 75% of the reciprocating as the balance factor to rotating mass. some companies use different factors but 75% is what i've seen as most common. so, 75% of 8gr=6gr against rotating mass. quite frankly, i don't think you'll be able to feel it at anything under 8Krpm. the reality of theses engines as Ancientdad pointed out, is an imbalance in the basic design built in by Honda. who knows, with the accumulated tolerances in the other bits stacked up, yours may run smoother.

just my 2cents.....
 
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