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Discussion Starter · #1 ·
Hey all, here's what I got.

I tore down a '78 CB400 because the compression was low, ungodly low. The poor thing has been parked in less than favorable conditions since '86. All that jumped out at me was a lot of tiny scratches at the gaskets. Looks like the PO had changed them and went overboard with a wire brush cleaning up. One side of the cylinder base gasket was blown at some point and filled with caulk. The piston on the caulked side was a lot grimier, but no obvious reasons for compression issues. Might have been rings?

Basically I am going to replace gaskets, rings, and seals. I just want to know if there is anything I should investigate before the rebuild? I don't want to put it together just to tear it down again.

Check my project build for pic's
 

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Discussion Starter · #4 ·
I knew the valves where a possibility. Is there a sound method to check before reassembly? My thought(hope) was replace what I knew needed replacing, rebuild, and test again.

As far as the cylinder wear, I'm a new wrench and assume since the buildup I found on the "grimy" side is only on the underside, maybe the problem didn't make it past the rings. "?" The engine's got 10K on her, and looked pretty clean, and the pistons had little clearance from the walls.

I don't plan to rebuild till Jan, the tear down was mostly to establish budget requirements. That said, I have plenty of time to do some reading. :geek:
 

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It'd probably be a good idea to lap the valves but I'm new to certain parts of these bikes myself.. Others with more knowledge will hopefully chime in soon.. :?:

I've heard of people turning the head upsidedown and filling them with acetone to determine how bad or if the valves leak. Acetone has less surface tension or viscosity than water. I'm assuming that's why they use it.

GB :mrgreen: :?:
 

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Bird76Mojo said:
It'd probably be a good idea to lap the valves but I'm new to certain parts of these bikes myself.. Others with more knowledge will hopefully chime in soon.. :?:

I've heard of people turning the head upsidedown and filling them with acetone to determine how bad or if the valves leak. Acetone has less surface tension or viscosity than water. I'm assuming that's why they use it.

GB :mrgreen: :?:
That's exactly right, Gerald.
I turn the head on it's front (or back), so the ports are facing straight up, then pour acetone into the ports - first the intakes, then the exhausts.
If there's a leak, acetone will find it.
If the valves don't leak at all, don't touch 'em !!!!!
Lapping valves well is partly an art, takes a lot of technique. It's the kind of thing that, if you don't do it very often, you can really make a mess of things. A good machine shop can do it perfectly every time - my guy only charges about $10 per valve, and I happily paid him for it on my 450.
Now, on Wifey's CB200 I lapped them by hand myself, but I was sweatin' like a rat at a Jack Russell Terrier convention. I had a spare head and valves just in case I screwed it up too badly. I hadn't done it in years and it took a while to get it right.

As for the cylinders, take them to a machine shop and have them miked - they won't charge much for this, a few bucks. If they're in spec just hone 'em and throw in some new rings.
One thing to keep in mind is the cylinder "taper", the difference in the bore from top to bottom. The allowable spec on this is very small, way less than 0.001". And it matters.
It's why I bored out my 450 project, the taper was about 0.0015".
 

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So you actually fill the ports and not the combustion chamber. Interesting.. I assumed you'd be filling the combustion "domes" much like they do when "CC'ing" a head..

This is good info. ;)

GB :mrgreen:
 

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Bird76Mojo said:
So you actually fill the ports and not the combustion chamber. Interesting.. I assumed you'd be filling the combustion "domes" much like they do when "CC'ing" a head..

This is good info. ;)

GB :mrgreen:

GB -

That's the way I do it alright, I pour the acetone right in the ports.
For one thing, a 450 head is very unstable on the bench, they wobble around really badly when they're upside down. And 450's have always been my main interest and specialty, so most of my techniques are geared towards them.....
And it's much easier to look at the valve from the combustion chamber side - you can see the stuff leaking around the perimeter of the valve right away.
And the combustion "dome" really doesn't hold much liquid in the first place, plus you'd have to put the spark plugs in, etc. You'd have to keep the head dead level, because the valves extend almost to the circumference of the "dome".

I've just always felt it's easier this way.

The acetone will dry up almost immediately, won't leave a mess at all.
 

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Cool. Thanks Bill! I'll be using your method (if my feeble mind can remember it) for my upcoming engine rebuild/refresh...

P.S. Did you see all of the Honda tools on Ebay specifically meant for servicing 450's?? Think I may snap up that valve stem punch. I wish I would've had the money for the seat cutter set the guy was selling...

GB :mrgreen:
 

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Yeah, the valve cutters were a find, but I couldn't afford them either.
The valve stem punch is a waste of money - if you heat up up the head first, the guides practically drop out with just a tap (usually).
 

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Disclaimer: my experience is limited to mower engines(many), VW(3), chevy small block(1), and my honda 100.

Regarding the valves - if you are going to have a machine shop check the cylinders, you may want to ask them to check and lap the valves in, and check the valve guides. I recommend you check the exhaust valves especially to see if any of the edges are "burnt" from the heat. The ideal valve wear will be a uniform ring around the center of the face of the valve corresponding with a similiar ring in the center of the seat of the valve.. Do the valves have rubber O-ring seals? if so they probably need to be replaced. Buy a valve spring compressor if you are going to do it yourself.

Rather than hand lap small engine valves, I have used a simple tool made by taking a shock absorber bushing, squeezing it between two washers with a nut on a 3/8" x 3" bolt. I attach this to a portable drill then add a little water and some valve grinding compound to the face/seat of the valve. Power the drill and let the bushing tool spin the valve from the edge of the top of the valve. Do not apply pressure in just one place and you can reverse the drill also to get a good seat. With a little practice on how much down pressure to apply, this has worked great for me. Be sure to clean out any and all compound when finished. Check the wear pattern when done. If you can get a drill on the stem of the valve, put some copper between the chuck and the valve and just use the drill to seat them. [If this is a bad way to do this for some reason let me know]


Make sure the grooves in the pistons are completely clean before you put in the rings. Look for any nicks in the piston that will cause the ring to stick. Be sure to stagger the rings (3 rings = 120degrees apart). Use a ring compressor if you can. I have used a metal can and a hose clamp but the right tool works best.

Larry
 

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Discussion Starter · #12 ·
Bird76Mojo said:
I'm new to certain parts of these bikes myself..
Your doing better than me, I don't know anything other than what I have read out of the Clymer... :?

I will certainly give the acetone a try, but for what tbpmusic says it cost him, I might be better off watching over the pro's shoulder at the local shop.

He runs a shop out of the garage, so maybe I can pester him while he works. After all, the reason I chose a clunker was to learn as much as I can from the rebuild ;)

Thanks for the help guys!
 

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Spinning the valves with a power drill as described is a really, really bad idea.
Your suppose to match the surfaces. Rotating valves, while it may look OK is actually wearing out seats and faces plus putting radial grooves making them wear much faster.
If you have to do it, use extra fine compound and only a few seconds lapping.(I have been known to use chrome polish)
I haven't actually lapped valves for many many years, if they leak its usually because the seats need cleaning up so I do that instead. Lapping valves is really an obsolete technology (you can't do it on just about anything from mid 90's on, and some bikes from the late 70's - 78 on) I'm pretty sure the CB40 T uses interference angles, (46 seat 45 valve) if the seats are cleaned up properly with a cutter they wont need any form of lapping
PJ
 

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CresentSon said:
Your doing better than me, I don't know anything other than what I have read out of the Clymer... :?

Stick around a while and you'll pick up on quite a few little tricks..

GB :mrgreen:
 

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Discussion Starter · #16 ·
I don't plan on going anywhere. Without you guy's all I'd have is ambition and a lot of official licensed Honda paperweights... :)

As far as the caulk at the base gasket, could that have allowed anything in that might have caused havoc in the Cylinder. If so would there be any signs other than cylinder wear? Logically i don't see it being the direct cause of the compression loss, but it might have been where the problem originated...

This is like a whodunit...
Thanks again all...
 

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I doubt it did anything "BAD". Its not going to let anything in on the compression side of the piston. If you dont see any scoring or gouging on the inside of the cylinder, your probably not going to have any damage up there because of it. It sounds more like a severe case of "too cheap-itis", as the PO probabaly didnt want to spring for a new gasket and went for the bath tub caulk "hi speed, low buck" fix.

You would be wise to take the cylinder to an automotive machine shop and have someone there measure your cylinders for wear. That would be the first thing I would do. You have to establish base line data in order to fix it right the first time and not go the "Well its probably just this" and have to tear it down again route.

My two cents.
Any others want to weigh in?

Henry
 

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Discussion Starter · #19 ·
Sounds like my best bet Henry, I am going to the mech sometime inside the next month, I'll have him spec everything out for me. He's got better tools anyway. He already knows that I insist on doing the wrenching so I can learn, but he can insure precision that I can't.

At least I picked your brains first, now I'll know what I'm talking about.

You guys rock :D
 
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