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Re-hashing the idea of a fuel efficient 4-stroke in an LST2

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Old 12-28-2013, 08:42 AM
  #1
1QwkSport2.5r
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Default Re-hashing the idea of a fuel efficient 4-stroke in an LST2

I have a .60 4-stroke engine I'm gonna attempt to stuff into an LST2. Engine redline is 13,000rpm, it will run without nitro really well and sounds really cool. When it came out back in the early 80s, it was the strongest .60 4-stroke on the market.


Now what I don't want to hear is the guys that will say it's not worth it, it'll be slower, it'll run hot, blah blah blah. I don't want to hear it. I have 2 LST2's, a spare engine, and the "why not" attitude.

The stuff I need to find to make this possible is obviously a new engine mount. I'll make a custom one. Next, I need a flywheel and clutch that will work with the existing transmission/spur setup. The truck is Forward Only already and hi/low range has been removed. The 4-stroke engine has a 7mm diameter crankshaft. The crank has a flat spot milled into it so a key can be made and used in the bore of the flywheel to keep it from slipping/rotating.

I can make the necessary modifications to make the throttle linkage work so I'm not worried about that. The factory LST fuel tank may need to get relocated higher since the carb will be higher but I won't worry about that until I can get the new engine mount made.

So really, I'm seeking advice as to what my options would be to fit a clutch and flywheel to the 7mm crankshaft. Worst case scenario, I make my own. I heard CEN's 7.7cc flywheel might work, or the MGT 8.0 flywheel might work. I just don't know for sure. Anyone got any ideas?
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Old 12-30-2013, 08:08 AM
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I have no idea, but I can't wait to see how this works. The LST is a great platform, should make for a nice sound from the truck too.
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Old 12-30-2013, 07:00 PM
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Some guy put an OS .70 4-stroke in an LST2, there's a video on YouTube. But it appeared that his engine had a geared unit to up the rpm at the clutch to better match what the 2-stroke could do. I don't want to get this crazy. The tough part is the limited gearing possibilities with the LST2 transmission. I need a bigger clutchbell gear and smaller spurs. With the right bearings, the stock clutchbell might work, but finding the flywheel that fits is the toughie. I need to know what the center bore is of the CEN 7.7 flywheel as well as the MGT 8.0 flywheel. None of the online retailers I looked at had any specs for the flywheel other than outer diameter and thickness.
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Old 12-31-2013, 08:54 PM
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Cool I'm doing a 4 stroke conversion on my mgt 8.0

First is that no there is no rc car clutch nut or pilot shaft readily made that'll fit a 6mm or larger crank you'll need to improvise

What I did with my magnum .70 which has a 5/16" crank is that I bought a try turn spinner adapter the shortest one and I threaded a 10-32 threaded rod onto where the nut that holds the spinner goes. Then I simply slided on the flywheel tightened the clutch nut and placed the clutch bell on it. I'm using a 16/19t clutch setup from a .15 small block engine...

This is just to get it running in the truck itself I plan on either machining the crank, getting an adapter for 1/4-28 which will fit the 8.0 clutch setup or simply getting a heli clutch but they are big in diameter.

For the mounting all I did was simply grabbed a pair of those generic engine mounts for .12 to .18 size engines and flip them upside down... The holes that grab the chassis were spaced enough that I just used a file to grind some material out of the engines mounting holes and bang perfect fit. You can barely see the work done.

Cooling will be done by belt driven fan. Gearing? Ah well I'm not there yet... With the stock 8.0 tranny I literally need to get a 1:1 clutch spur ratio to achieve the same speed as the 2 stroke. But if I can't get a clutch spur 1:1 ratio then I'll have to find some bevel gears and tinker with the diff.

One idea I also had was placing a 2speed assembly where the spur is on the 8.0 tranny to make a 3 speed.

This is still a work in progress, no rush for me. And no it's not a bad idea as long as you have adequate gearing you can outrun a 2 stroke...

The 8.0 clutch is 1/4" and the cen is If I remeber correctly the same or slightly smaller metric like a 6 or 5.49 something I'll check the basement I have 7.7 stored
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Last edited by Barracuz; 12-31-2013 at 09:07 PM.
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Old 01-01-2014, 05:40 AM
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I appreciate your help. A 5/16" shaft is really close to 7mm in diameter. I use the same props on the .60 4-cycle that I use on a big .90 2-stroke. The .90 is 5/16". One thing I'd seen is how far the clutch is to the front housing of your engine. My engine has a longer front housing on it as it is, so I need it to be as close as I can. I probably will need to bore out a flywheel and mill a key way into it to have as short of a crank as possible. The crank has a milled face so the prop driver will not slip. I will get a few more pictures of the parts and the truck when I get closer to being able to take it apart.

Likely I'll need a negative gear ratio but where is a guy gonna find a clutchbell with two pinions on it with more than 26 teeth on it. Right now I'm running 19/26 on the clutch and 63/70 spurs on the 2-speed.
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Old 01-01-2014, 08:23 AM
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Well I'm pretty sure you get a smaller spur... Buku or werks sells 38t metal spurs probably the smallest commercially available. But if you can mill your own flywheel why not mill the clutch as well? And if you do make plenty as the engine WILL strip teeth.

There was this guy bout 4 yrs back who made the drive hub into the flywheel... I think it's the same guy who made the lst2 with the gear reduction unit just a different project.
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Old 01-01-2014, 01:05 PM
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I don't have a mill, I'd be modifying parts for other vehicles to fit. If I can find a flywheel that I can modify to fit the engine (to take the place of the drive hub) I'd be in good shape. I should be able to use a 3 or 4 shoe clutch and a factory flywheel with different pinions. The hardest part will be finding a flywheel to work.

I need to to pull the truck apart and try to find out if the engine will even fit. I'm not sure if it will yet.
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Old 01-08-2014, 01:47 AM
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For the flywheel, your not going to find the dimensions of the hole. However you may have more luck looking up the dimensions of the flywheel collet. But I've thought of doing this too and I don't think you really need them. Leaving the engine's current flywheel on does make things longer but it's also a square surface for the LST flywheel to sit against, no collet required and no cutting a key way into the flywheel either. The only foreseeable problem is the size of the nut the engine came with being too large and interfering with the clutch shoes and springs. Finding a more suitable nut shouldn't be too hard if that's the case though.

Another point to to consider is turning down the crank to 6mm. You don't need a lathe for this. Simply chuck up the Dremel with a cutoff wheel or sanding drum, mount the engine in a vise, start it, turn the Dremel on and slowly turn down the crank until the bearings fit. With the engine running and the crankshaft spinning the crank will still be perfectly round. I and others have done this with brushless motors turning down 6mm shafts to accept 5mm pinions and you're only removing .5mm of material. The hard part is getting the diameter even as it requires lots of checking with bearings. I recommend using a marker to mark where the diameter is good or bad so you don't go too far.

You do have some options in the gearing department. First is the swap or flip mod. Inside the transmission you simply take the gear on the input shaft and swap or flip it with the gear it meshes with. Some light sanding of the trans case will be required. This will make your 2.09:1 LST2 trans and make it a 1.91:1 ratio. The other is to buy the bevel gears that Losi includes with their electric conversion kit. I figured it out once but don't remember the exact numbers, I think the ratio ends up around 1.6:1 when combined with the swap mod. Lastly, RC-Monster.com sells the gears that thread onto the threaded LST clutchbells so you can customize the ratio. The biggest they have is 25t for 1st and 30T for 2nd gear but Mike does make custom stuff if you ask and need something bigger.

Last edited by Maj_Overdrive; 01-08-2014 at 03:13 AM.
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Old 01-08-2014, 03:07 AM
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After looking at the CEN 7.7's crankshaft you can see the front bearing and the flywheel mounting surface are the same diameter. After more searching around I finally found the front bearing size of the CEN 7.7 and found out its .375" (9.52mm) inside diameter according to Boca Bearings.

The Mgt 8.0 flywheel isn't going to help you either. While the 8.0 uses a different flywheel than the 4.6 (4 shoe vs 3 shoe) the flywheel collet is the same so both share standard SG dimensions.

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Old 01-08-2014, 05:34 AM
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One more thing you'll need to keep in mind is how to start this engine. The easiest way would be to have a spinner after the clutchbell and use a typical airplane starter. The other option is a boat flywheel or custom boat style flywheel with a groove in it for a belt so you can use an airplane starter. Food for thought. Keep us updated and definitely get some pics of the engine's crankshaft up.
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Old 01-08-2014, 07:20 AM
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Thanks Maj or the info. Couple things: the engine cannot be modified in a fashion that will prevent it from flying a plane later. So turning the shaft isn't gonna happen. Finding bearings that fit is the route I'll need to go. Also starting the engine without a prop or flywheel on it isn't wise because it needs the flywheel effect from a prop or flywheel to keep it going. This particular engine needs a rich idle mixture so that flywheel effect is needed to keep the fire lit; the cam/valve timing is pretty hot so the idle isn't dead smooth either. This engine isn't made anymore either, so parts are hard to find. If I could a spare crankshaft for it, I could modify it for smaller bearings but again parts are hard to find.

If a regular flywheel will fit the 7mm shaft, it can be shimmed to fit tight and an adapter but can be used as to not interfere with the clutch shoes. As to the gear flip mod; already have both LST2's with this mod and also already FOC. I just need the engine to fit and turn a clutch that will turn the spurs and still shift. The rest can be adjusted to make it work. I've had a death in the family recently so I've had no time to look the LST over and determine what I need to do. Life should slow down some now so I can get some stuff done.
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Old 01-08-2014, 10:17 AM
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Not modifying the shaft makes it a little tougher but not impossible. I understand what you mean about the flywheel effect and I never meant to imply removing any weight, except turning down the crank. The piece of aluminum (flywheel?) that's on the crank near the crankcase would stay. I meant to say the truck flywheel would go right up against the airplane flywheel with a nut on the clutch shoe side to keep it there. But if modding the crank is out the truck flywheel will likely have to closer to the end of the crank. This can be done easy enough with a couple of nuts. But nuts won't keep the truck flywheel very square like the airplane flywheel would so I'd use a collet to keep the truck flywheel square to the crank. Normal .21-.32 big blocks are all the same as far as collet inner diameter, unfortunately I don't know the exact size, just that it's bigger than the 5mm of normal clutch bell bearings. Maybe you'll get lucky here.

Btw I ran some calculations and the gearing situation doesn't look good. Assuming 12,000 rpm, gear flip and E conversion bevels (1.6:1 is what I put for trans ratio), 30t 2nd gear, 63t spur and stock LST tires top speed is about 23mph. Finding bearings may be a pain, but figuring out how to get better gearing without the jack shaft overdrive in those vids is going to be just as tough.

Sorry for your loss. The hobby will be waiting when you have the time for it.

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