$txt['youtube'] = 'YouTube'; $txt['youtube_invalid'] = '#Invalid YouTube Link#'; Side by side comparisons of the EX21, EX17, & KX21 head's and blocks

Author Topic: Side by side comparisons of the EX21, EX17, & KX21 head's and blocks  (Read 5630 times)

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Offline 2mokaiman

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Just some observations I made with the EX21  EX17  KX21 engines I thought I'd share with you all. I finally got around to dismantling the ES - EX17 I bought for the starter and flywheel with gear ring for the spare KX21 pull start engine I purchased in 2013 on Tom's site. I also had dismantled an EX21 & a KX21 to swap the KX21 head over to the EX21.  Putting them side by side I could finally see the physical differences in each one.I will post pictures so all can see the differences. From left to right is the EX21 EX17 & KX21. The reason I put the EX17 in the middle is is to show why Tom's swapping the EX21 head with the EX17 head got him that increase in speed and power. At the same time you can see how similar the EX17 head is to the KX21 which is what I run. The EX17 head in the combustion chamber is really filled in. There is very Little surface area to compress the fuel mixture compared to the EX21 which is open for the whole piston head. If you look closely at the valves on the EX21 & the EX17 you will see the # 7 stamped on the Exhaust and 7 stamped on the Intake. However when you look at the KX21 valves they have the # 5 stamped on the Exhaust an 5 stamped on the Intake. You will notice they are 1/8" larger diameter than the 21 & 17 valves.  The bigger valves are necessary to accommodate the much larger carburetor the KX21 uses, which I will post side by side pictures as well.  You will also notice the rest of the head over the piston is filled in with very little surface area for the fuel mixture just like the EX17. This results in much higher compression ratio from 8 to 1 with the EX21 to 10 to 1 on the EX17 & KX21. All 3 engines use the same 2" diameter pistons. The EX21 block is used for the KX21. The only difference between these 2 engines is the head and carburetor. The flywheel and ignition coil are identical and the same for the EX17 as well. Hence me buying the ES - EX17 for that reason.The Cam gear is the same for all 3 as well. The only difference between the EX17 block and the EX21 block is the EX17's piston cylinder is 7/16" shorter resulting in a shorter stroke thus less HP. I have a picture with me holding a combination square across the top of the EX17 block and running into the side of the EX21 block to show the difference in the cylinder length. Up until today I really didn't know why the EX17 was only 6hp. Now I know it has a shorter block resulting in a shorter stroke. Everything else is identical to the EX21 with the exception of that modified combustion chamber on the head.
« Last Edit: March 02, 2015, 12:59:55 AM by 2mokaiman »

Offline Painlesstom

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Excellent post Tim, thanks for taking the time to share all of that with us. Very interesting that it is stroke, and not piston size, that made the difference between 17 and 21 models. I learned something new today.  \./8
Tom

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Offline Odie

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Tim, I haven't gone into my kx yet. Are the kx-ex cams exactly the same?

Offline 2mokaiman

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Tim, I haven't gone into my kx yet. Are the kx-ex cams exactly the same?
Hi Rick, I was fortunate to find a pdf file with the EX21 & the KX21 part #'s side by side for comparison. let me know if this is helpful.
http://www.r-kart.com/pdf/201404parts_catalog.pdf

Offline Peter

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there is a difference between the EX and KX - flywheel key is cut at different spark ignition timing 11 degrees

Offline Painlesstom

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You are correct Peter, there is a timing difference between the KX21 and EX21. But, Tim has used the KX flywheel and the EX flywheel on his KX motor and has noticed no difference in performance. These motors were originally intended for go kart use at high 5600 rpm, but being used in the Mokai the best you will see is around 4500 rpm, and I don't think the extra timing would do anything there.
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Offline Odie

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I'm familiar with the timing differences. Thanks Tim, I've got a hard copy of the catalog somewhere? Nice to have the PDF on my tablet now too. There is a difference in the cams according to part numbers, what exactly needs some research. I'll try to cross reference numbers with other rs engines. Seems (from vague memory) the decompression mechanism was changed to accommodate the higher cr. I'm mainly interested in the cam profile comparison. I know the Aussie kart sites offer a performance cam profile for ex21's with 17 heads to bring the hybrid engine closer to kx performance. For certain the kx, ex21, & ex27 each have different part numbers, in fact, my rs catalog indicates two different cam part numbers for the 21, maybe super ceding numbers?
In reality, the kx is pretty much a hybrid with the unique exception of the head casting. That's why I would not be surprised if the kx cam exists on another rs engine.

Offline 2mokaiman

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I'm familiar with the timing differences. Thanks Tim, I've got a hard copy of the catalog somewhere? Nice to have the PDF on my tablet now too. There is a difference in the cams according to part numbers, what exactly needs some research. I'll try to cross reference numbers with other rs engines. Seems (from vague memory) the decompression mechanism was changed to accommodate the higher cr. I'm mainly interested in the cam profile comparison. I know the Aussie kart sites offer a performance cam profile for ex21's with 17 heads to bring the hybrid engine closer to kx performance. For certain the kx, ex21, & ex27 each have different part numbers, in fact, my rs catalog indicates two different cam part numbers for the 21, maybe super ceding numbers?
In reality, the kx is pretty much a hybrid with the unique exception of the head casting. That's why I would not be surprised if the kx cam exists on another rs engine.
Rick I have 4 KX21 engines and 2 EX21 engines. I checked this morning and all the cam's are identical. Just like the one I have pictured beside the EX17, They all have the # 277 stamped on them. So like you said the # difference listed in the catalog and the pdf. file must be related to the decompression issue not the profile.

Offline Odie

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Good to know Tim. With electric start the decompression mechanical differences, if any, should be mitigated substantially with electric start. On the other hand, my pull rope with an ex21 /17 head or kx wasn't particularly fond of the 10+ cr. Cheap rope and wrong pull angle probably...