Help with CM200 Engine (Compatibility) with CM125

[NICK4298]

Hi All I have just purchased a 1983 CM125 custom is very nice condition but it has had a cm200cc engine replacement, As my son has not passed his test he is not able to ride this as its been reregistered as a 200 Therefore can i replace the head barrels and pistons with a 125 topend is it compatible with the 200 bottom end, I would prefer to keep it as a 200 but dont think there is any other way round apart from doing his direct access which is too expensive at present, Any ideas are welcome thanking you in advance kind regards Nick

[nightshaddow]

short answer - no

the cm 125 and 200 have different strokes with the 200 being longer, if you fit the 125 barel and pistons to the crank you will come across 3 main problems

1st the pistons on the 125 hold a 13mm gudgeon pin the 200 is 14mm

2nd the barels them selfs will be too small to directly fit the crank case making gasket placement and oil flow more difficult to line up

3rd if you managed to work round it and get passed the first 2 you would end up smashing the pistons into the head

my advise would be get a second hand 125 lump and fit it keep the 200cc for a later date

tldr

short answer - no
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Current rides CB 125 TDE(the next project) NS125RK ( now on the road ) CB125TDC (soon to be project 17) melody ( the nephews )