Vintage Dirt Bike Q & A

Hi,

I am hoping someone can better expalin this to me, I have a friend that just bought a cr250m and he tells me that it has a rod kit installed in it, between the base of the cylinder and the lower end, it has an aluminum spacer installed, I rode the bike and it was a lot smoother powering thru the gears than I expected, it was more like a modern CR than the older ones that I have rode in the past. I am very close to starting my 74 cr 250m project and was wandering if someone might be able to offer a little more info on the Rod kit ( if thats the correct terminoligy for it) and maybe a suppier that could hook me up with some parts.


Thanks,

Claude

Sounds like a stroker kit was installed. I never saw one on a two stroke,I am interested as well.

It does sound like a longer rod has been installed, but this can be done with and without increasing the actual stroke. In both cases the cylinder needs to be raised some to accomodate the greater distance between the center line of the crank and the top of the piston at TDC. The latter approach does not change the capacity, but changes the port timing on two-strokes. I seem to remember the YZ250s run by the factory team in the early 90s had "long-rod" motors. I assume this was allowed within the AMA "production" rule.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

Check AHRMA.org for the ad but some guys in Ohio have the complete rod kits for the 73-74 CR250 for a decent price. The plate is because the rods are a couple of mm longer and the plate makes them work. The rods are much more expensive from Honda and they were out of lower pins last I checked. If you can even find the exact rod kit it will probably be much more expensive. I got three of these kits and used one so far and it works great. Power is only slightly affected by the plate. If your rod is not too worn you might just replace the lower bearing which is expensive by itself.

Check with badbrush.com for those rod kits.

When you say Power is slightly affected by the plate is that in a good way or bad?

Would it effect top end, low end??

Thanks

Hey, just talked to my buddy again and he explained things a little better, just like one of the post says, the rod is about 4 mm longer than the stock rod which actually strokes the bike somewhat, which mellows it a little and made it more evenly powered from low end to top end. I cant really explain the difference other than the bike wasnt as radical and as pipey on the top end as a stocker. just right for an old fart like me that has broke enough bones for 2 lifetimes!!!!!!

Doesn't actually stroke/increase displacement unless it came with a new crank. Probably lots of 'do good' in it, just not from displacement.

I thought I would jump in here and explain the long rod setup a little further. First thing, as a couple have said, it does not in any way change the stroke of the engine, nor change the displacement of the engine. The stroke is controled by the pin location on the crank. The longer rod changes the pistons speed in different parts of the cylinder, for example, the longer rod over the stock rod will travel slower at top dead center and bottom dead center, and travel at a faster speed in the middle of the stroke. As a general rule the longer rod will loose a little top end and bottom end power and pick up power in the mid-range if nothing else is changed. The long rod is also a little easier on parts because of it's slower speed when it changes directions at top dead center and bottom dead center. I have built a lot of engines with both length rods and it all boils down to what type of engine the rider wants. The long rods have been used in the 73/74 elsinore for a long time, the biggest reason to start using them some time ago was because there were no rods available in the stock length either from Honda or aftermarket.

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Sam Alexander

1974 Honda CR250M
1976 Honda CR250
1974 Maico MC440
1974 Maico MC250
1977 Maico AW400