Where can we purchase a new motor for our 21A-643B066 tiller?

Question edited for a lot more clarity.

Is it like this?

https://www.amazon.com/Pro-Chaser-21A-340-129-21A-332A500-21A-332C765/dp/B07WDHJ7Y3

https://www.google.com/search?q=21a-333a129+air+filter

..

First, I'm not a Yard Machine representative.
* check your warranty. Call for service.
* Web site; https://www.mtdproducts.com/equipment/mtdproducts
* another site; https://www.mtdproducts.com/webapp/wcs/stores/servlet/category2_10500_20500_1341950_1341884_1341884_-1

*** The following is for information only.***
* Powered equipment requires maintenance.
* Remove power cord when checking blades.
* Basic electrical operation of tiller is Power cord-Power cord/socket-Switch(safety lock out)-Fuses(or breaker)-Trigger Switch-Starter capacitor(or starter control)-Motor.
* Common issues with electrics;
* Cord cut/twisted in tiller.
* Bent or burnt plug or connector. (look for black points on plug blade. Check for burnt smell)
* Examine cord for rubbed spots (from pulling around rocks)
* Examine cord for "Melting".
* Check if the cord is rated (and how long). Longer runs cuts power.
* Disconnect power to tiller. Check blades for obstruction. Remove mud and rocks.
* Check if Tiller blads are locked up. They should move with some resistance from motor. (gas tillers; blades should turn with clutch disengaged)
* Check trigger for "snappy" click or free movement for operation. (some models will lock unless another leaver or grip is pressed to release trigger.)
* Check intermediate cord for damage (some models hae an exposed cord to motor head that can get caught or cut)
* Note; following sugestions require opening up equip which will void warranty. If your not sure then check first with your store or vendor.
* Parts diagram; https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/mtd/yard-machines/21-tillers/2010-models/y155-21a-155a900-21a-155a900-y155/general-assembly
* Parts; https://www.repairclinic.com/Shop-For-Parts/a24b324d1032061/Model-Y155-Yard-Machines-Tiller-Parts
* Customer's repair (similar; Earthwise tiller); https://www.youtube.com/watch?v=ybnp6IJMGbI
* Customer's review of Earthwise Tiller; https://www.youtube.com/watch?v=HcshAmP9zZQ
* Lady's review of Sun Joe TJ603; https://www.youtube.com/watch?v=S_xTLzzTZHo
* Suggested maintenance(Mantis) https://www.youtube.com/watch?v=K81ljLHtvSs
* Still have problems? Call for service.
Aloha, ukeboy57

PS; Additional info
* Customer's first time for changing oil; https://www.youtube.com/watch?v=LadsrFjIJxs
* Customer's TB154 electric operation; https://www.youtube.com/watch?v=Af5-IXK74A8
* TB154 (official) Set up; https://www.youtube.com/watch?v=xziHxA634uQ
* Customer checking purchase of used M/W gas tiller; https://www.youtube.com/watch?v=XWtS3bIuBvY
* Powerhorse gas Tiller fix; https://www.youtube.com/watch?v=lAPpkqv_YtI
* Customer compare gas vs electric; https://www.youtube.com/watch?v=GUVyO7eEWZs
Forums;
* https://www.mytractorforum.com/214-tiller-talk/1145810-yard-machine-mtd-rear-tine-tiller-21a-422b129.html
* https://www.dirtdoctor.com/efl/dirtdoctor/root/topic3447.html

A safe gap is the width of a business card

If you need further help, I’m available over the phone at https://www.6ya.com/expert/kelly_69a0cf1b13789df1

in the engine 10w-30 in the gear box i think 80w-90

Please see part number 26 on this website's page. This belt if for your exact model. The part number is listed and you can also purchase it from here if you like.

http://www.yardpartswarehouse.com/parts-look-up.html?arismn=12227#/Troy-Bilt/21A-512-081_Tuffy_Tiller_12227%2c_12227_Tuffy_Tiller/Power_Train_Assembly/003620-C/0036200016

The drain plug is generally located on either side of the engine near where the block bolts to the frame of the tiller. It can be a square headed plug or an allen socket head plug.

Yes, the Tecumseh engine takes SAE HD30 motor oil. With a complete drain, it should take about 21 ounces to fill.
Good luck!

Possible broken key where crankshaft meets gearbox.

I went to MTD web site and they had all the manuals for all there products. You will need a model and serial number to get them. Thank you for responding back to me.
I got the manual for the tiller and the engine.

The pdf manual can be found at mtdproducts.com. You need to know your model and serial number to access the download. Really not that good a manual.