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Discussion Starter #1
I have a C21CPN commercial, remote filter, no BBC or other gizmos.

The engine has a lot of end play. I like the stagger deck and large fuel tank, its been a good mower. My son is taking on mowing some family yards and I want to re-power the mower so we don't have to be concerned it will fail during the growing season.

Is there a drop in replacement engine?
 

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Discussion Starter #4
Any "F" engine should bolt to the muffler plate. It would be best to pick an engine from a similar year model.
I think an F engine is going to be hard to locate. I was thinking about a new engine. Honda, Kawasaki, B and S, etc
 

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I think an F engine is going to be hard to locate. I was thinking about a new engine. Honda, Kawasaki, B and S, etc
Well personally I think it will be much easier to do like what was already suggested, and that is to just find a good replacement F series shortblock rather than converting to a new 4 cycle engine. Your C21CPN was only made for two years (1989 and 90), the closest Lawnboy model to that in a 4 cycle (B&S) is a 680531 so you would need to bump those two parts catalogues and come up with the parts that you would need to covert it over to the 4 stroke new engine setup. Just a quick scan and Partstree is showing that the engine plate (pn#614429) that your new replacement 4 stroke would mount on is NLA, so that would have to be sourced used from ebay, or a similar source. The Briggs engine part number for that model is (pn# 126702-3192-01) is also showing no longer available, so a replacement with the same mounting pattern would be needed. Just a big head ache in my opinion for very little return, as a matter of fact, I think you will end up with a machine that is not as functional, or as desirable as the old original setup of the F engine. Now I don't know, but there is a chance that someone makes an aftermarket adapter plate for these old Lawnboys so that these new 4 stroke replacement engines can be mounted, but I have yet to hear about it. Once again, it would be my humble opinion that you would be spending money and time actually downgrading your highly desirable F engine equipped mower if you were to follow through and get rid of that F engine capability. There are still plenty of good new and used F series engines out there to be had. Heck, you may get many more years from that original engine if the only issue is crank end play. I forget the measurement off hand, but the manual gives quite a bit of lead way on crank end play, don't quote me, but .015 in. comes to my mind, so you could locate a good shortblock and stuff away until needed. It's an easy change over to when the time comes. You have a highly desirable mower as is. I see a constant ad being run down here in CL in west Texas of folks wanting to buy old 2 stroke Lawnboys, especially the commercial versions.
 

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Depends on the year of your LB but most can be repowered either by a direct bolt or with a home made adapter plate. In the pictures is my LB commercial with a Briggs via an adapter place. A LB 10201 With a Harbor Freight engine. A JS60 John Deere with a a HF Predator and I have probably 5 or 6 other mowers that have been repowered. The HF engines I have been real happy with and those will be around for years to come. Roger
 

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If you go with a new engine I would recommend the B&S Quantum engine as that's what was stock on 4 cycle models from that era. The mounting plate is available on eBay and from experience the Quantum engines are plenty powerful. Not to mention Briggs parts are a little easier to come by.

Link for the mounting plate: https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.ebay.com/itm/NEW-NOS-LAWN-BOY-LAWN-MOWER-PLATE-UNDER-ENGINE-PN-614429-DISCONTINUED-PART-/123736658607&ved=2ahUKEwiWwsuv9Z7iAhVMM6wKHfWvCgkQjjgwAXoECAYQAg&usg=AOvVaw0xIn_a9TD0ZaXcSDA5pETw

Sent from my XT1710-02 using Tapatalk
 

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Why mess around with adapter plates and a four stroke adding a bunch of weight,just replace with an F.Parts aren't that hard to find,besides the OP is looking for a drop in replacement.
 

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Discussion Starter #9
Thanks for the replies.

I would prefer to find a NOS F engine for replacement but I know that will be difficult. That is why I wanted to look into options for a modern(currently in production or recently out of)engine.

I will look around on Craigslist as well as talk to the former Lawnboy dealership. Maybe I will get lucky and find something. Otherwise I will explore the adapter options.
 

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Another vote for another F engine. This isn't LB territory like the mid-west I bought an L21ZPN and a 4 square JC Penny for 30$ Both in very rough shape. I got L21ZPN running just by taking the carb apart , soaking the parts and re-assembling. I removed the coil, wire brushed and sanded everything. It is an '89 and my old 7073 is an '88 The mowers are virtually identical with Walbro metal carbs. The engine has a lot of end play. I'm no LB expert. Generally speaking. End play in most engines isn't a big deal. Axial play is the bad one.
 

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I have been trying to remember something about a thrust washer that was added to the F series to help eliminate the end play crankcase wear on the F series. Also from what I have read, the bearings on the F series are a bit on the loose side as far as lateral movement. I would look for a back up mower to use if the Lawnboy fails down the road. Keep an eye out for a short block on eBay. They do come up. That way you have all the bases covered. Besides, no one that mows should only have one mower. :) Bill
 

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I have been trying to remember something about a thrust washer that was added to the F series to help eliminate the end play crankcase wear on the F series. Also from what I have read, the bearings on the F series are a bit on the loose side as far as lateral movement. I would look for a back up mower to use if the Lawnboy fails down the road. Keep an eye out for a short block on eBay. They do come up. That way you have all the bases covered. Besides, no one that mows should only have one mower. :) Bill
Yes those crank thrust bearings came along very late in the F series engine lifespan. The 10401 that I bought my father in the mid 90s came stock with it.
 

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Lawnboy77, Does that mean my 10515 has the thrust washer? Good to know. Thank you. :D
Unfortunately the roller thrust bearings were never included in the F engine of the 10515. The last year for the 10515 was 1995 and the crank thrust roller bearings were first introduced in 1996. If you look at the parts catalogue for a model 10401 made in 1996 you will see two different versions of that F engine. The one produced early on that year (serial # up to 6901660) was a standard F engine. The serial numbers from 6901661 and up included the roller bearings, so that was a production change starting in 1996, which here in the U.S. was pretty much the end of the F series engine production. It would of been nice had all the F series had those roller bearings. I have never tried to mix and match thrust bearings with non thrust bearing crankcases though. I'm fairly certain that would require some machining to make room for the washer and bearing (approx. .062 in thick, if memory serves me correct). That thrust bearing set is still available for around 18 bucks from Partstree.com. Personally I've never had any problems with excess play of any kind in an F series engine, but I've never ran these in a commercial environment either.
 
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