[Alex jb]

My CD175 is called Tidler, it's a great little bike and can pull well enough for fun on a Sunday ride, however, there's always room for more and the bike was always as much a tinkering toy as transport so I got an engine from a CB250N superdream and will be tearing it down and teaching my son (and to some extent me) about the internals of late 70's tech Honda combustion before we find a way to get the considerably larger 6 speed 250 in the place of a 4 speed 175.

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[Alex jb]

The swap is not going to be simple, but if I wanted simple I would have left the engine alone or opted for a CB175 and maybe re-bored and jetted.
I think the CD175 with its single carb and 15hp could be raised to low 20's with a CB175 head/carbs and some machining/ tuning work.
But, the Superdream 250 engine was 27hp out of the box, 6 gears and is the same bottom end as the 400 also.
The 250 is a stressed member in the bike so if it really doesn't fit well into the CD frame, I may have to remove the belly of the CD frame.
The rear upper mount is 85mm wide on the CD and 110mm on the SD, the distance between the upper and lower mounts are also out so the CD box section will be in for quite a bit of work.


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[longdistancerider]

This is going to be interesting. Lots of pictures needed

 

[Simo]

Whats the sproket alignmemt like? If the SD is overall larger its likely that the front and rear sprockets wont be in a straight line

 

[Alex jb]

Whats the sproket alignmemt like? If the SD is overall larger its likely that the front and rear sprockets wont be in a straight line

Yes chain line is going to be yet another problem, but I have built bicycle wheels several times before and as in the twin leading shoe thread, I plan on re-lacing a front wheel over the winter time use a larger TLS hub than the 140mm dia SLS hub in the CD. Maybe I will have to source a different rear hub also.
The distance from the rear sprocket to the swing arm is quite generous right now, could accommodate a 20mm wider spacing easily


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[Boomer343]

Just a suggestion ....... clean up the exterior of the 250 engine then take it apart to see if it is a good engine. If it is good then leave all the internal parts out and reassemble the cases, cylinders, heads etc. Leave off any side covers for now.

Engine rebuilds are easy but doing a retrofit will have that engine tossed around 20 times more than what you think it will be. I spent many hours putting a Suzuki S40 650 engine into a Norton Feather bed frame with a Triumph 650 Swing arm conversion. That engine got to be my nemesis in more ways than one and if it had not been a very low mileage engine I would have gutted it for sure.

Also getting a laser that puts out a line to check alignments takes a lot of the guesswork out of things.

 

[Lefty]

Yes chain line is going to be yet another problem, but I have built bicycle wheels several times before and as in the twin leading shoe thread, I plan on re-lacing a front wheel over the winter time use a larger TLS hub than the 140mm dia SLS hub in the CD. Maybe I will have to source a different rear hub also.
The distance from the rear sprocket to the swing arm is quite generous right now, could accommodate a 20mm wider spacing easily


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I got leftover dirt bike wheels I could measure when the time comes.

 

[Simo]

The easiest approach would be to put the correct cb250 hub on the rear, and machine spacers to align it. If the hub is to wide to center youll need a plan b and c. theres no swing arm that will fit the cd frame that is wider at the hub that I know of (and ive spent a long time looking) To fit a Gb250 rear hub onto a cb200 frame Ive had to modify a cb125 swing, cut down the hub and cush rubbers, and build an offset front sprocket

 

[longdistancerider]

I have a couple of bare frames here that I can take pictures and measurements of as needed,

 

[Alex jb]

Started to have a look inside the SD engine, from the cam it looks very low mileage

Head off, started cleaning up a bit, bores looks good

one of the exhaust valve not looking so hot

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[longdistancerider]

Looks to be in fair shape. Note that the valves cannot be resurfaced, replacement only.

 

[Alex jb]

Need a valve spring compressor and think.

These 4stroke engines are funny, there's one too many piston rings too


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[Alex jb]

The other (possibly more important) reason for the SD engine tear down.

Bonding time with my boy

The pistons are in pretty good shape after Charlie cleaned them up

Want to get tr rotor off next but not sure how to lock it up an oppose the torque
Thought about a bar through the con rods but don't want to damage them

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[Simo]

Would the sd head and jugs fit on the cd cases?

 

[longdistancerider]

The rotor looks a bit different than what I'm used to, could just be the angle. Anyway it should have a pair of holes in it. By placing a 3/8" extension (female end) in one hole you can create a lever/fulcrum between that and the socket for the rotor bolt using a large/long screwdriver or such

ignore the text in the picture

 

[Alex jb]

I get ya,
I found this retro pic of a 'universal tool' in the 250T manual

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[Alex jb]

Would the sd head and jugs fit on the cd cases?

There's a remote possibility I guess.
According to wiki, the 250 N is 62.6 mm bore × 41.4 mm stroke
Only 0.4mm more stroke than the 175...


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[Alex jb]

Had to go to the garage and check,
SD jugs are 120mm centres and 66mm deep mount face to head gasket (inc gasket).

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[Alex jb]

SD cam gear is a different type of chain (many more teeth) with a 34mm spigot to fit on

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[Simo]

Ill head to the garage when im home