After returning from my summer long cross country trip, my CB500 engine was in bad shape. We'd lost 1st gear completely and I had a leaking head gasket. I have a stuck engine and had planned to combine my good parts and make a good engine again, with a 1st gear! Well, a cb500 engine popped up on craigslist for $50 nearby, and I thought it was worth going after. When I got it I saw it was in (apparently) nice shape, though I didn't know the history or compression numbers. Everything looked nice and when I took the valve cover off the cam lobes and bearings looked nice too.
So, after practicing engine removal by taking the stuck one out of my parts bike, I swapped my new engine into my bike and it ran great! Sounded great, was oil tight, smooth, everything! Great score right?! I went for a few short rides and took it to work.
Well last Wednesday night on my way home with the g/f riding on back, we had just left somewhere and I revved it up to 4k or so and I heard a most horrible clanky sound. At first I thought a branch had caught in the spokes so didn't think much of it, but the sound didn't go away immediately, instead it continued for about a minute. Things smoothed out and the bad grinding stopped, but the motor wasn't running right and I realized I only had 3 cylinders working. I finished the 8 mile ride home and began the investigation.
I got home and figured out it was cylinder #2 that wasn't running. I pulled the plug and oh my, the thing was mashed! I had never seen anything like it. When I explained the incident to my dad he was sure something had been sucked into the intake and ended up in the combustion chamber. Something smashed that spark plug to bits.
Well I pulled the valve cover and found all the valves held in place where they should be (my dad told me to look for a broken valve retainer). I attempted a compression check but couldn't because pulling that smashed plug out did a number on the spark plug threads. I could see through the spark plug hole, though, that something wasn't right inside that cylinder.
So, having never been below the valve cover on any engine, I proceeded to tear down the top end. Here is what I found:
Soooo..... a broken piston and severely damaged valves and seats in that cylinder. The exhaust valve had been broken to bits, when I pulled the head I dropped it because the end had broken off! And the last pic shows the remnants of something metal in the intake manifold.
My dad shook the muffler expecting to find pieces of a bolt or something, but all that came out was a piece of that broken exhaust valve. Still, I think his theory of an uninvited metallic object coming into the combustion chamber makes sense; when I pulled the air filter out there was a bolt and nut in there that was left over from when I had installed the engine!
So it's a mystery to me what happened exactly, but what is clear is that the top end needs to be rebuilt. So here are some newbie engine rebuilding questions:
1. First off I cannot get the cylinder off for the life of me. I've whacked it with a rubber hammer, put a screw driver in the provided pry-points, and even tried a method I found here of stuffing some rags in the cylinders, re-attaching the head and trying to kick start it. When I did this, not only did nothing move but I feel I nearly broke the kick start mechanism.
2. Since the combustion chamber of that head is toast, I'll have to toss it. Sad because the cam bearings looked real nice. I have two other cylinder heads to choose from, and am wondering if the cam bearings turn out to be no good in them, can they be replaced or reground? The manual refers to this, but I don't know if they mean replace the bearings or the head itself. Here's what it says:
"Inspect the camshaft bearing surfaces. Camshaft bearing surfaces should be smooth and shiny. If it is scratched or excessively worn, it should be replaced."
3. I want to do a re-bore while the cylinders are off. There are some kits on ebay for reasonable prices, but I don't know how much over I'll need to go. How do I figure this out? Currently there are .25, .75, and 1.00 over kits for around $150.
4. Are the honda ring compressors specific to the model? I happened to find two honda ring compressors like they show in the manual, but with a different part number. I don't know how much bigger or smaller one could be and still be useful for this bike.
Those are all my questions for now. I'm sure I'll have more. Perhaps I'll post a project thread and concentrate questions there. If you can't already tell, I've never rebuild an engine of any kind, but I've done tons of searching here on the forums and have a pretty good idea of what I'm doing. We'll see!
Jeremy B
