Gas-Diesel engine swap question

[handirifle]

I have a 20hp lawn tractor that has a Briggs and Stratton flat twin (flat head) 20hp, 750cc engine. It's got a valve seat on the exhaust side of one cylinder, that I have jury rigged back together. It's help for 4-5 mowings (3 acres) so far, but lately it kinda sounds like it's getting a little worse each time.

There is NO short block, long block or direct engine replacement for it, period. Even a similar sized replacement engine, that will have to be modified (the tractor most likely) quite a bit, to mount and set the exhaust system, possibly the fuel system too.

My only option is really to replace the mower, and am NOT going to buy another used one, so looking at about 3 grand for suitable replacement.

OK, that said, I was wondering how much horse power would be necessary in a diesel to replace the 20hp gas. I know it would operate at a slightly lower RPM, but I figure higher torque would offset that some.

I ask this only because I see the import 10hp engines on fleabay ar $600 or so, shipped. That would be worth it, IF it didn't cost a fortune to make the adaptation. The closest gas engine that Briggs and Stratton recommends is $850 plus shipping and tax.

 

[3238dpw]

what is it going in?

 

[handirifle]

Oops. forgot to mention that. It's a Murray riding mower with a 52" cut. Large rear tires and is a perfect size for my needs, but I think the engine is doing well if it's mustering 12-14hp in current condition. Maybe less, but no way to tell. I haven't done a compression test lately, that might give me an idea, but it's kinda like I don't really want to know. Ignorance is bliss you know.

Since there is no further fix for the main problem, it's just a matter of running her till she gives up the ghost.

 

[mjncad]

How old is the mower? Any way you go, you could easily be throwing good money after bad if you're not careful.

 

[oldnslo]

That old of a Murray, what shape is the mower deck in? Seems like they have a tendancy to rust/wear out where ever the grass hits them. I realize $3000 is a lot of money but is sticking $500 or more into existing a good investment? I suspect that the time to mount a diesel engine and get all the pulleys etc. to fit and line up properly may make this task a real challenge.

Engine running at lower RPM will require diffent drive pulley size to get same blade speed. Is there room for a larger pulley? WIll the belt guides and tensioners clear?

ROy

 

[Monster5601]

OK, that said, I was wondering how much horse power would be necessary in a diesel to replace the 20hp gas. I know it would operate at a slightly lower RPM, but I figure higher torque would offset that some.

A diesel rated at 18 HP @ 3200 RPM (typical RPM for a small diesel) would provide about 30 ft.lbs. of torque which is equivalent to your gas engine assuming it delivered its rated horsepower at 3600 RPMs.

As mentioned, your are going to need to adjust the blade drive-line to increase the blade speeds.

 

[handirifle]

All good questions and feedback. An 18hp diesel would more than likely drive the cost up well beyond what would ever be considered prudent, I am afraid. Took it out this AM wioth the trailer to pull my tree watering buckets, and it ran like total crap, so hopefully there will be no more mowing required this season, cause I don't think this engine is up to the task.

I figured pulley speeds would be different but didn't consider clearance issues. There are some tight areas so looks like this idea heads to the scrapyard as well.

As for the condition of the mower and deck, everything else is in very good shape. No rust at all on the deck, under or on top. I thought it was gonna be a real good buy this spring when I got it for $500, oh well, live and learn.

 

[David_Kb7uns]

Murray lawn mowers are a dead end for parts in the long run. Everything made before the bankruptcy is no longer supported also .

David Kb7uns

 

[Iplayfarmer]

$500 isn't too bad for that big of a mower if the rest of it is in good condition like you stated. If you're considering going to the effort and expense of a diesel engine for this one, you might consider keeping your eyes out for a similarly sized mower with a good engine and bad everything else. Even if you found a 14 to 16 hp engine it would likely work. The effort to swap engines is nothing compared to the amount of adapting and changing that you'll have to do for a diesel. Around here, old mowers can be had for cheap to free.

 

[Treemonkey1000]

I think if you create a template of the deck mounting holes or the engine holes and find a reasonably close replacement motor any decent machine shop can make you a mounting plate to line up a replacement motor. Exhause doesn't have to be pretty and fuel lines don't vary that much. That's if you really want to spend the time. Otherwise put it on Craigslist for some other person that may need the chasis that has a good motor..Then take the money and go find another mower that fits your needs.