Mechanics porn: Honda RC166 engine teardown - Wrist Twisters
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post #1 of 8 Old 04-15-2017, 03:38 PM Thread Starter
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Mechanics porn: Honda RC166 engine teardown

I have been practically obsessed with understanding the engineering behind the legendary RC166 250cc 6 cylinder engine ever since hearing one run on the Wide World of Sports in 1967 (I think), and accidentally came across this documentary including a teardown of the engine on youtube:
. It's all in Japenese, but that makes little difference to a lifelong mechanic!

No further commentary on my part: just enjoy, and if you have any questions I can probably give you answers based on my over 50 years of studying and working on engines, and as much information as I could find on the RC.

Rob
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post #2 of 8 Old 04-15-2017, 03:40 PM
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post #3 of 8 Old 04-15-2017, 04:53 PM
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I bet that thing would have really screamed with a K&N filter and Ram Air.

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post #4 of 8 Old 04-15-2017, 05:14 PM
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....and some fresh Rotella...
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post #5 of 8 Old 04-16-2017, 06:21 AM
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Not a 166, but an exploded view of a RC161 engine hangs on my garage wall.
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"Towards the end of the vid, it looks like she may have had a bafflectomy." - MarylandMike
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post #6 of 8 Old 04-16-2017, 12:55 PM
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The engine is like a high jewel count mechanical watch of old.
Superlative machinery.
Interesting use of the Oldham coupling for the two piece camshafts, let alone the barrel shaping of the cams.
Then there's the 12 piece crankshaft, and it's not welded up after fitting like 70s era Superbikes were.

What I am not at all clear on though, is reference I found elsewhere to cylinder specific valving and lobing of the cams.
I'm guessing is that the barrel shaping of the cams dictated a range of base circles and associated valve stem lengths.
Rob, do you know something about this particular aspect?

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post #7 of 8 Old 04-18-2017, 01:38 PM Thread Starter
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Quote:
Originally Posted by mcromo44 View Post
What I am not at all clear on though, is reference I found elsewhere to cylinder specific valving and lobing of the cams.
I'm guessing is that the barrel shaping of the cams dictated a range of base circles and associated valve stem lengths.
Rob, do you know something about this particular aspect?
The primary reason for the hollow barrel section cams was to decrease rotational inertia to facilitate faster response to the throttle as well as keeping weight down, especially above the CG. It worked too: a dry weight of only 112 KG (247 pounds), and a throttle response so fast that an incautious application of the throttle in neutral would send the revs past redline in less than 1/2 second, potentially floating the valves quickly followed by getting fired.

As to the cam profiles: unfortunately you can't just shrink the cam profile for cylinders 3 and 4 to suit the smaller diameter cam body. The base circle, quieting ramps, flanks, and nose would be wrong for the slightly heavier valves, requiring a complete quintic refiguring of the profiles. Add to that the springs would have to be a slightly higher rate as well, usually by raising the spring seats in the head and decreasing the spring length by as little as 1mm. A hassle, especially when servicing the motor between races!

Another aspect to consider is the cam followers and the bores they work in. If you look at the followers in the video you would almost certainly notice they are not cup shaped as is universal in more modern direct acting DOHC motors, and while they look solid such could not be the case: they would be far too heavy, and heavy in a valve train that spins 10,000 RPM is the last thing you want! Extrapolating from my experience with a 1961 CR110 motor, the followers were hollow and astonishingly light, about half that of a single CBR600 follower! The only way I can think of achieving such a light weight is to make them hollow (duh!) with wall thickness less than 1mm and a post down the center to accurately transfer the cam motion to the valves. As to valve adjustments -- there were no shims of any sort. You had to carefully measure the clearance, then remove the cams and followers, measure the follower, place it in a special tool that held it accurately in a slide above a flat plate on which you placed a strip of sandpaper, and sanded the tip that engages the valve until it was the right height to give the clearance you need. Go too far and place it in a bin in preparation for a rebuild later in the season, and start again with another follower. You may well ask why they weren't cups. Simple. There just wasn't enough room to fit springs inside a cup that small. Also, while I have no direct information on this, it is logical to assume the follower blocks were different heights in order to maintain full engagement of the followers in their bores. A teardown required what you see in the video: wells for each part to make sure you don't have to carefully mike each component so it goes back in where it came from.

On another aspect of the motor, in this case the crankshaft, which incidentally required a multi piece pressing fixture that weighed more than the whole motorcycle, is the lubrication of the roller connecting rod bearings. You would assume, as I did, that it was as high tech as the rest of the motor. NOPE! It uses the same method of lubrication as your lawnmower! That's right: splash lubrication. Look at the video and you will see a rod journal drilled down the middle, and a catch piece machined on the end to get oil slung from the main bearings. Of course the bearings require little oil, in fact too much will cause problems such as increased drag and skating instead of rolling, but it was quite a surprise to see. All this, and innumerable other facets, from the fertile mind of one man: Soichiro Irimajiri, the astonishing genius responsible for most of Honda's racing motors in the '60s, and incidentally the CBX. No surprise there.

Rob

If it has already been done, it is safe to assume it is possible to do it.
On the other hand, if it has not been done never assume it is impossible to do it.
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post #8 of 8 Old 04-18-2017, 04:09 PM
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Quote:
Originally Posted by robtharalson View Post
The primary reason for the hollow barrel section cams was to decrease rotational inertia to facilitate faster response to the throttle as well as keeping weight down, especially above the CG. It worked too: a dry weight of only 112 KG (247 pounds), and a throttle response so fast that an incautious application of the throttle in neutral would send the revs past redline in less than 1/2 second, potentially floating the valves quickly followed by getting fired.

As to the cam profiles: unfortunately you can't just shrink the cam profile for cylinders 3 and 4 to suit the smaller diameter cam body. The base circle, quieting ramps, flanks, and nose would be wrong for the slightly heavier valves, requiring a complete quintic refiguring of the profiles. Add to that the springs would have to be a slightly higher rate as well, usually by raising the spring seats in the head and decreasing the spring length by as little as 1mm. A hassle, especially when servicing the motor between races!

Another aspect to consider is the cam followers and the bores they work in. If you look at the followers in the video you would almost certainly notice they are not cup shaped as is universal in more modern direct acting DOHC motors, and while they look solid such could not be the case: they would be far too heavy, and heavy in a valve train that spins 10,000 RPM is the last thing you want! Extrapolating from my experience with a 1961 CR110 motor, the followers were hollow and astonishingly light, about half that of a single CBR600 follower! The only way I can think of achieving such a light weight is to make them hollow (duh!) with wall thickness less than 1mm and a post down the center to accurately transfer the cam motion to the valves. As to valve adjustments -- there were no shims of any sort. You had to carefully measure the clearance, then remove the cams and followers, measure the follower, place it in a special tool that held it accurately in a slide above a flat plate on which you placed a strip of sandpaper, and sanded the tip that engages the valve until it was the right height to give the clearance you need. Go too far and place it in a bin in preparation for a rebuild later in the season, and start again with another follower. You may well ask why they weren't cups. Simple. There just wasn't enough room to fit springs inside a cup that small. Also, while I have no direct information on this, it is logical to assume the follower blocks were different heights in order to maintain full engagement of the followers in their bores. A teardown required what you see in the video: wells for each part to make sure you don't have to carefully mike each component so it goes back in where it came from.

On another aspect of the motor, in this case the crankshaft, which incidentally required a multi piece pressing fixture that weighed more than the whole motorcycle, is the lubrication of the roller connecting rod bearings. You would assume, as I did, that it was as high tech as the rest of the motor. NOPE! It uses the same method of lubrication as your lawnmower! That's right: splash lubrication. Look at the video and you will see a rod journal drilled down the middle, and a catch piece machined on the end to get oil slung from the main bearings. Of course the bearings require little oil, in fact too much will cause problems such as increased drag and skating instead of rolling, but it was quite a surprise to see. All this, and innumerable other facets, from the fertile mind of one man: Soichiro Irimajiri, the astonishing genius responsible for most of Honda's racing motors in the '60s, and incidentally the CBX. No surprise there.

Rob
Thanks for the time taken to make such a detailed response.
There was lots of great info.

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