Might be a long post to read, but trying to chronicle what I have done with this revo so others may learn what does/doesn't work more easily. In my few weeks of research, I didn't find many lengthy posts on the 21TM and how it really stacks up to the 3.3 and 18TM. Just a lot of "get the THS pipe and the BUKU pipe is AWSOME AND YOU HAVE TO GET THE M2C CLUTCH!!!! and OH MY GOD THIS IS THE AWESOMEEST ENGINE EVER! and YOU SHOULD BUY A PICCO 26!!! " type posts.
After a short lived "what do people think of the OS21TM" thread, I knee jerked and bought one on ebay for $250.
I was looking for something with more grunt than my 3.3 was giving me, which I was never really impressed with. I have a second big block leaner kit and a NIB picco .28 sitting in a drawer, but I really didn't want two full fledged big block revos. I liked having the small block revo as it was quieter, lasted longer on a tank, things didn't wear out as fast... but, after a 2 or 3 gallons with the 3.3 I put in a couple years ago, it just felt like a slug compared to my BB revo. Ran hot, felt like it could barely get out of it's own way, finicky tune (fixed by putting my 18TM carb on it), but never felt like enough power for the bashing I do. I was running 38/15 gearing with standard ratio trans.
For kicks, I found my old and tired 18TM that has probably 7+ gallons on it, tore it apart, cleaned it and installed it so I'd know exactly how a 21TM compares to the 3.3 and 18TM. It had been sitting in a drawer neglected for 3+ years and was all gummed up. I ran it pretty hard for a day last week just to remember what it was like. I don't believe I ever ran it in a 3.3 chassis revo, so it wasn't nearly as wheelie happy as I recalled it to be, but it still ran a lot better than the 3.3 and seemed a much better fit power wise. Occasional wheelies in the grass, ran pretty cool (230F), idled nice, ran nice, started easy... perhaps I bought the OS21TM needlessly.
It was too late, I had already bought the 21TM and it was on it's way.
Installation of the 21TM was pretty easy, using the same integy 2 shoe 7075 shoe clutch with traxxas flywheel, 15T bell, 38T spur for all 3. Also using the same THS 1046 small block high torque pipe. Currently the only revo pipe offered by THS (they have 3 really, but all the same tuned pipe with different look, chrome, anno'd and standard). When I bought it 2+ years ago, they had 2 small block pipes, one for higher rpm and one for low end. I was trying to coax more power out of the 3.3, so went for the low end pipe. With the header gasket that OS supplied with their revo header that came with the 21, I was able to coax the header onto the engine after lubing the seal a bit and gently working the header over it. It's thinner than the THS gasket.
I'm running the THS 146B in my LRP28S3 revo with a highly modified header as the THS header sucks with the New Era leaner style mount.
Initial break-in went/is going ok. I only had the 125cc tank when the engine showed up and I heat cycled the first 3 tanks (run the engine for about 5 minutes with it tuned to give temps around 230F, shut it down, BDC the piston, cool, redo adjusting tune to keep temps up). For tanks 4-8, I ran through each tank entirely, still gently running and keeping temps up, BDC, cool between tanks. I got the 150cc tank before running today. I spent one tank running around a parking lot, still being pretty gentle with it, then went to a different location and ran tanks 11-12 through it a bit more aggressively.
Now it's still not "BIG BLOCK" power, but it's definitely getting stronger with each tank. Pulling a pretty easy wheelie in the grass and with a bit of effort, will do so on pavement as well. I only got through 3/4 of tank 12 and the main input gear sheared it's teeth on me while I was punching the gas for wheelies and ended my day. It was running around 230-240F throughout and I needed to lean it quite a bit from last weekend to keep the temps up. The engine is clearly wearing itself in and not struggling the pinch as much.
So, the first 21TM victim is my two speed gear:
How the engines compare side by side for size:
My rig:
Will post up how the engine is doing as I get more fuel through it. I'm currently stuck running odonnel 20/30 mixed for 25% nitro, 9-10% oil I think. I'm also using the LC3 plug that came with the 21TM. I will more than likely pick up some LC4 plugs as it will probably help keep the temps down by getting the timing better with the 25% fuel I run.
So far, the engine shows promise and hopefully will be a happy medium between the 3.3 and a true big block. I'm hoping I'll be able to run in the grassy parks I have more easily without overworking the engine too much. Was that worth $250... I don't know, but so far, it doesn't seem to be a bad decision. Odds are, I'll also upgrade the trans to RRP steel. If possible, I'll buy the input and output gears only, but not sure if RRP sells them separated like they used to for the t-maxx as I'd like to avoid as much rotating mass as possible.
After a short lived "what do people think of the OS21TM" thread, I knee jerked and bought one on ebay for $250.
I was looking for something with more grunt than my 3.3 was giving me, which I was never really impressed with. I have a second big block leaner kit and a NIB picco .28 sitting in a drawer, but I really didn't want two full fledged big block revos. I liked having the small block revo as it was quieter, lasted longer on a tank, things didn't wear out as fast... but, after a 2 or 3 gallons with the 3.3 I put in a couple years ago, it just felt like a slug compared to my BB revo. Ran hot, felt like it could barely get out of it's own way, finicky tune (fixed by putting my 18TM carb on it), but never felt like enough power for the bashing I do. I was running 38/15 gearing with standard ratio trans.
For kicks, I found my old and tired 18TM that has probably 7+ gallons on it, tore it apart, cleaned it and installed it so I'd know exactly how a 21TM compares to the 3.3 and 18TM. It had been sitting in a drawer neglected for 3+ years and was all gummed up. I ran it pretty hard for a day last week just to remember what it was like. I don't believe I ever ran it in a 3.3 chassis revo, so it wasn't nearly as wheelie happy as I recalled it to be, but it still ran a lot better than the 3.3 and seemed a much better fit power wise. Occasional wheelies in the grass, ran pretty cool (230F), idled nice, ran nice, started easy... perhaps I bought the OS21TM needlessly.
It was too late, I had already bought the 21TM and it was on it's way.
Installation of the 21TM was pretty easy, using the same integy 2 shoe 7075 shoe clutch with traxxas flywheel, 15T bell, 38T spur for all 3. Also using the same THS 1046 small block high torque pipe. Currently the only revo pipe offered by THS (they have 3 really, but all the same tuned pipe with different look, chrome, anno'd and standard). When I bought it 2+ years ago, they had 2 small block pipes, one for higher rpm and one for low end. I was trying to coax more power out of the 3.3, so went for the low end pipe. With the header gasket that OS supplied with their revo header that came with the 21, I was able to coax the header onto the engine after lubing the seal a bit and gently working the header over it. It's thinner than the THS gasket.
I'm running the THS 146B in my LRP28S3 revo with a highly modified header as the THS header sucks with the New Era leaner style mount.
Initial break-in went/is going ok. I only had the 125cc tank when the engine showed up and I heat cycled the first 3 tanks (run the engine for about 5 minutes with it tuned to give temps around 230F, shut it down, BDC the piston, cool, redo adjusting tune to keep temps up). For tanks 4-8, I ran through each tank entirely, still gently running and keeping temps up, BDC, cool between tanks. I got the 150cc tank before running today. I spent one tank running around a parking lot, still being pretty gentle with it, then went to a different location and ran tanks 11-12 through it a bit more aggressively.
Now it's still not "BIG BLOCK" power, but it's definitely getting stronger with each tank. Pulling a pretty easy wheelie in the grass and with a bit of effort, will do so on pavement as well. I only got through 3/4 of tank 12 and the main input gear sheared it's teeth on me while I was punching the gas for wheelies and ended my day. It was running around 230-240F throughout and I needed to lean it quite a bit from last weekend to keep the temps up. The engine is clearly wearing itself in and not struggling the pinch as much.
So, the first 21TM victim is my two speed gear:
How the engines compare side by side for size:
My rig:
Will post up how the engine is doing as I get more fuel through it. I'm currently stuck running odonnel 20/30 mixed for 25% nitro, 9-10% oil I think. I'm also using the LC3 plug that came with the 21TM. I will more than likely pick up some LC4 plugs as it will probably help keep the temps down by getting the timing better with the 25% fuel I run.
So far, the engine shows promise and hopefully will be a happy medium between the 3.3 and a true big block. I'm hoping I'll be able to run in the grassy parks I have more easily without overworking the engine too much. Was that worth $250... I don't know, but so far, it doesn't seem to be a bad decision. Odds are, I'll also upgrade the trans to RRP steel. If possible, I'll buy the input and output gears only, but not sure if RRP sells them separated like they used to for the t-maxx as I'd like to avoid as much rotating mass as possible.
Last edited: