Honda Twins banner

1 - 20 of 33 Posts

·
Registered
Joined
·
25 Posts
Discussion Starter #1
Hey guys, I haven't posted in a while but I've been lurking in preparation for the top end rebuild of my 1980 CB400T. When I got the bike running last summer, I knew it had plenty of blowby but I got a good year out of it until now, when I figured it would be a good time to freshen it up. The engine has 32,000 miles on it, and I'm guessing it hasn't been opened up in a while, if not since it was built. The camshaft and rockers look good, and there was plenty of oil on them. The pistons and heads had significant carbon buildup but I'm getting them cleaned up. I did the acetone test on every valve and they all appear to be sealing. What I intended to do was slap on a new set of rings and call it good, but when I gave the cylinders to the machine shop to have them honed, they inspected them and told me they were in pretty rough shape, I think they even used the word "terrible" :eek:. Visibly they looked okay, but they found a bunch of high and low spots in them. They suggested I had them bored, and I planned to get some oversize pistons and follow through with it, but today I found this: https://www.davidsilverspares.com/CB400T-1980-HAWK-USA/part_175324/. If I get this, I will be able to use the new standard size rings that I already have, and I may even be able to use my old pistons (I'll get them measured but I'm not holding my breath on that, considering the cylinders were that bad). I'm fairly sure the new cylinders (even with new pistons) will be cheaper than a bore and hone at the machine shop, and that's not even adding the cost of the oversize pistons. This is my first time doing anything this in-depth with an engine, so I'm asking you experienced people, does this seem like a good idea or would you rather go with the bore?
 

·
Registered
Joined
·
2,266 Posts
That cylinder has been modified to use the CB 450 oiling, and who knows what other mods. It might be bored oversized also. I think I would just have the old cylinders bored, and get a piston kit with new rings, wrist pin, and C-clips. Now is not the time to be frugal. You will end up spending dollars to save dimes.
TOOLS
 

·
Super Moderator
Joined
·
24,208 Posts
The modification to the cylinder is as described. The original 400 cylinder has smaller, slightly, bolt bores particularly the right inner holes. Originally those 2 bolts did not have the rubber on them to prevent corrosion of the bolt. In late 1980 the bolt were changed incorporating the rubber and the cylinders were modified for clearance. Other than that they are identical to the 1978 on cylinder.
I question the machine shops description of high and low spots. Cylinders are measured for taper and out of round. I did my engine at 60K, the cylinders were in spec and the pistons were in spec still, worn but in spec.
For the price of that cylinder I would go ahead and get it, have the pistons measured to be sure they're good, light hone of the cylinder to clean it up, toss in the rings and be done.
 

·
Registered
Joined
·
25 Posts
Discussion Starter #4
Hey, thanks guys! I was researching the oil flow of the 400 and 450, and while I couldn't find the flow diagram of the 450, it looks like on the 400 the cam stuff is getting oil from the outside rear bolt passages. Correct me if I'm wrong, but even if the new cylinder has different right inner holes, it shouldn't make a difference regarding oil flow, right? As for the rubber on the bolts, the bolts I removed from those holes did in fact have the rubber boots on them, so I'm guessing the old cylinder I have is the new design, or the "modification" as described on the the site. The diagram on CMSNL for the old CB400 cylinder tells me it's obsolete, and that it's been superseded by a new design with the same part number as the new cylinder I plan to get. TOOLS1, it appears that the CB450 gets rid of the oil holes in the outer cam journals and uses a little pipe instead to oil the cam, but this shouldn't have an effect on the oil flow of the 400 since the cylinders appear to be the same aside from a larger bore and a different base gasket that would cover where the oil orifice would be on the 400. Again, I could be wrong.
 

·
Super Moderator
Joined
·
24,208 Posts
You are correct on everything. The 400 get it's head oiling from the outer main bearings thru the orifice and up the rear outer bolts. The 450 does away with that and instead is getting the head oiled from the center main bearing which in my opinion is not good since that center main is also feeding the transmission, both balancer's and the left rod. The one advantage the 450 head oiling has is the intake rockers are pressure fed with cam lobe squirt holes in them for better cam oiling, that's why I modified my 400 head to incorporate that feature.
 

·
Registered
Joined
·
2,266 Posts
I thought the rubber on the bolts was to block off the oil on the 450, and that is why they had the oil fed from a tube on the outside.
TOOLS
 

·
Registered
Joined
·
25 Posts
Discussion Starter #7
So after looking at the pistons again, there's some pretty significant scoring that I didn't notice at first. So I need new pistons, no problem, except I can't find any. Every fiche just says it's been discontinued except for CMSNL, but they only have one, and they are $83 a piece (I know what you said TOOLS about trying to save money, but man...). The eBay seller cruzinimage_co has every size of piston for $52 a set, except for the standard size which appears to be sold out. I guess I'll keep searching but if anyone knows of somewhere I can get my hands on some new pistons, I would greatly appreciate it!
 

·
Super Moderator
Joined
·
24,208 Posts
Contact the ebay seller cruzinimage and ask when he expects more. He's been the only source recently for stock sizing. I've looked at the pistons and they seem to be decent quality, member oldschooldropout bought a set.
 

·
Super Moderator
Joined
·
24,208 Posts
Just how bad are the pistons? You say scoring, pictures will help in this case. There may be an alternative to replacing them depending on the damage
 

·
Registered
Joined
·
25 Posts
Discussion Starter #10
Here's what I am dealing with, I don't know how I didn't notice it before
 

·
Registered
Joined
·
25 Posts
Discussion Starter #11
I asked cruzinimage about it, Mitsuo told me that the pistons wouldn't be available until around November. I'll keep my eyes peeled for anything I can find because I don't really want to wait that long. Of course I could just have then bored and such, but I'll try at least a little harder to find some standard size pistons.
 

·
Registered
Joined
·
25 Posts
Discussion Starter #13
Thanks for your help Jim, I'll probably have to use different sources for each piston unless I can find a set
 

·
Registered
Joined
·
25 Posts
Discussion Starter #14
Well I guess the machine shop honed the cylinders anyway, even though I told them that if they measured out of spec, I wanted them to call me and *not* do anything with them. They probably didn't measure them at all, they most likely honed it first and then they saw the "high and low spots" that they described to me. I guess that was a waste... :roll:
 

·
Registered
Joined
·
2,266 Posts
Honing them is the quickest, and easiest way to tell if they are going to need bored. That is the first thing I do when I pull an engine apart. You would have a lot of time involved in trying to measure for high, and low spots. Remember "Time is money". You are going to have more invested in this by doing it piece meal then you would by byuing the oversized piston kit that comes with Pistons, rings, wrist pins, and C-clips.
TOOLS
 

·
Registered
Joined
·
25 Posts
Discussion Starter #16 (Edited)
Huh, that makes sense, I just pictured them using a bore gauge or something and at least checking before they honed them. I asked them about it and they explicitly told me that they would measure it first before they did the hone. The way it's looking now, I might spend less to use the old cylinder. It still *probably* will be significantly cheaper to do it "piece meal" using a new cylinder than it would be to bore it, but it will be harder to find pistons since the supplier of the piston kit you are talking about ran out of standard size pistons a couple days ago.
 

·
Registered
Joined
·
2,266 Posts
Since you need to bore it, you will need oversized pistons, so it does not make any difference if he does not have any standard pistons. Going standard with the new cylinder, you will end up buying a piston here, a piston there, then rings, then you will need new wristpin C-clips, and you should replace the wristpins as well, and after that you will need to find the different bolts... Just get a 0.5mm over kit, bore your cylinders, and be done with it.
TOOLS
 

·
Registered
Joined
·
25 Posts
Discussion Starter #18
I already have new rings, circlips, and the right bolts, it's just a matter of finding one other piston and maybe some new wristpins
 

·
Registered
Joined
·
25 Posts
Discussion Starter #19 (Edited)
I did some fancy mathematics and I determined...
Getting the old cylinders bored with new aftermarket piston kit: $170.50
Getting new cylinders, NOS standard pistons, new circlips (I already have new rings): $148.90
Parts shipping is included
So, it's not a HUGE difference but it is noticeable. New cylinder option would have been a bit cheaper but like I mentioned the aftermarket piston supplier is out.
 
1 - 20 of 33 Posts
Top