CR250 Cylinders

[Gerry MacNutt]

I'm wondering about the difference in cylinders from 2002 to 2004 that are now being sold by Honda. It seems the 2002 comes with the power valve but not the 2003 and 2004. But they all have different part numbers, Here are the numbers for the "A" sizes.

2002 12010-KZ3-L20 includes power valve
2003 12110-KZ3-L30 no power valve
2004 12110-KSK-670 no power valve

So is a 2003 the exact same cylinder except just no power valve?
Also has Honda improved on the design of the power valve that would come with a new replacement 2002 cylinder?

[Ian Harrison]

Hi Gerry

[Gerry MacNutt]

Thanks you very much, yes that was very much helpful. As you know the parts images do not represent accurate images for some of the differences between certain years.

Thanks very much!

[Dan Davis]

Hey Ian! Hope all is well with you!

With regards to the 2004 cylinder. When you say "extra work" I assume you are referring mostly to the horrible exhaust port on that cylinder?
I ask because I am blessed with a 2004 cylinder and yes, it did take plenty of work to get the exhaust port into reasonable shape and also did a fair amount of work on the transfer ports as well.
Otherwise, are there other detrimental characteristics of this cylinder when compared to the 02-03? I am installing this cylinder on 2005 cases by the way....I know, probably not the most effective route to travel but it is what it is. Everything is coming on the cheap ya know.

I've set the 04 cylinder up to run with domed 2005 piston & ring with outer edge of piston approximately .050"/1.27mm (approx. squish clearance) below the cylinder deck with approximately 196 deg. of exh. port timing, and around 134 deg. of transfer port timing. All just best guesses for effective use of the trial and error process knowing full well I'd likely be better off on track by simply purchasing a cylinder/2 piece head (and pipe?) package directly from Viper.....But then there is much less fun & stress in the off season using that method and most likely less fun and additional stress on track. Sadly, this is my nature.

Since I am shooting for least desireable hodge podge of parts from various years , , I ended up with a stock 2003 cylinder head. I will be cutting the mounting surface and outer edge of the squish band flush with the deck of the cylinder and will be increasing the CCV of the from stock a bit to boot. I'll also be cutting the head for o-rings once I get the combustion chamber in shape.....another challenge as the outer o-ring gland won't be concentric with the center of the combustion chamber....enter an NC mill or rotary table /manual mill.

I'll be using a 2005 CR VF3 reed valve (which also took a fair amount of work to modify for strait intake system) and will be fabricating an adapter to accept a Mikuni style intake rubber....I have modeled a couple of iterations via computer software but haven't yet created an actual part for that task as of today.

Any thoughts?.....Possibly my entry here combined with your experienced feedback could be of some help to others as what to do or not to do with these engines. Please try not to fall off your chair due to hysterical laughter, I don't want to be responsible for possible injury!

Perhaps this would be better suited for posting in the "All things CR250" forum.

[Ian Harrison]

Hi Dan

Great Post!! and good to hear someone getting stuck in and making the best of things!!

In terms of the 2004 cylinder, yes it's the exhaust port with it's flattened top and base that takes a fair bit more carving, however the results can be just as good as the earlier cylinders, just that Gerry was looking at new, so I would be inclined to buy the one that needs the least work to get to your end result.

Conventionally, I would say it's fine to mount the '05 cylinder on '02-'04 cases but the other way round does offer a bit more obstruction of the inlet tract than you would like. But if it's what you got . . . . .Must admit because visually it didn't look too good I never tried it, so I can't actually say how much that costs you in power terms, it may well be smaller than I imagine.

Your figures of 196/134 are good, but I'm surprised that you've got 196 deg exhaust port duration with a piston recession of 1.27mm. I'd have to check when I get in work, but I'm pretty sure that we get 196 with a piston recession of 2.3mm. We then have a head step down the cylinder of 1.0mm that gives us a 1.3mm squish. Perhaps you raised the top of the exhaust by 1mm or perhaps the ports are set a bit higher in the '04 cylinder. Anyway suffice to say, your port timing figures are were they need to be. I'd be interested to know what spacer/gasket combo you have under the cylinder. We would normally have about 2.7mm total thickness to achieve our 2.3 recession, so I'd expect you to have about 1.67mm of gasket/spacer.

Of course, if your cylinder had been skimmed 1.03mm off the top at some time and you were running 2.7mm spacer/gaskets that would also explain it

When using the dome piston, just make sure you have sufficient angle on the squish ramp, so that you don't end up with a negative squish! (getting smaller towards the centre) It's easy done. Just ask Mr. GasGas !!

All the '02-'04 heads have the offset chamber (of course, to match the cylinders) so we have a jig to turn them. Have fun with your one-off, we've been there

Your inlet sounds good, just a lot of work (of which your obviously not afraid), but I do believe that the Mikuni 2-bolt rubber is a bit stiff for a CR250 and particularly on a rough track. I did come across these from my development archive. Just some development shots from when we (in the UK), weren't allowed to remove the lumps and strengthener web at the back of the kickstart shaft. I fabbed up that angled alloy adaptor with hand tools and a pillar drill (I know . . . it shows!!) Those were the days:



Anyway, good luck and please share anything else we may find interesting . . . .carb, ignition, pipe, etc. but, yes, in the all things CR250 topic (10,8504 views and counting!!). The originator of that thread Erik Frank would be smiling at those early inlet shots. My friend, Erik actually made the first (I believe), V-Force 2 adaptor for the crankcase reed CR250 (and made 1 for me). That's it on the left of shot with a special rubber, I had made to replace the defunct RS125 angled rubber!

Happy days

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Dan Davis]

For more info concerning the last two posts on this thread, see page 27 of:

http://www.ekartingnews.com/viewtopic.php?t=66607&start=390