My OS 21TM 3.3 revo

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olds97_lss

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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:
2015-1024-RevoOS21TM-FriedTopGear-Issue01.jpg


How the engines compare side by side for size:
2015-1019-TRX33_OS18TM_OS21TM_004.jpg

2015-1019-TRX33_OS18TM_OS21TM_002.jpg

2015-1019-TRX33_OS18TM_OS21TM_003.jpg


My rig:
2015-1019-RevoOS21TM_001.jpg

2015-1019-RevoOS21TM_002.jpg


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.
 
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olds97_lss

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Well, sheered off a pin in the trans on one of the primary output gears mid-way through tank 3 today. Ordered RRP steel gears for $72 on ebay.

Appears the plastic may have warped, then bent and sheared the pin. A piece is still stuck int he gear sideways.
2015-1025-OS21TM-RevoFail02.jpg


Hoping the steel gears will be less likely to cause this issue. This pin sheared off the other day with my 18TM as well.

Engine was running a bit weird compared to yesterday. Not sure if it was the fuel, my tune, the engine or just an off day. Power was spotty and temps were kind of erratic. Will have to give it a good once over to see if I can spot any leaks. Included a LC4 plug in my order. Just hope the 50F+ temps hold out for a few more weeks so I can get more wheel time on this before it's too cold to run a newer engine.
 
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olds97_lss

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I've broken two 2 speed shafts in half in my BB revo. They break where the first output gear pin is. At least in that thing, it made sense, but this failure doesn't make any sense. I replaced that gear/pin recently... no wait, that was my bb revo.
 

Cat

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Stripped 2 pinion gears in my BB revo, after that i went for RR Steal gears.Never had any problems with the gears. Even if the slipper is to tight there is no problem. I was afraid of loosing some top speed due that steel is heavier to turn then the plastic.
 

olds97_lss

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Well, so far not overly impressed with the 21TM. Took it out again today and ran a few more tanks through it. Seems to run on the warm side. Even with 65F temps out, it was running at 260F with a pretty rich tune. I didn't do a lot of off the line throttle jabs to avoid breaking my trans again. So far, it's a hair better than the 18TM. Probably over 1/2 gallon through it so far. Not sure I'll get much more time running it this year as I really don't want to run it when it's below 50F outside since the engine is so new.

The pin I broke in the trans is the one that is held in with that god awful c clip thing. I didn't have a spare, but I had some hardened steel pins I bought many years ago for cvd pin alternatives. I had to put it in a drill and grind it down to get the diameter right. I just wanted to run it a bit since it warmed up.

I'm guessing the temps will drop some and power will be a bit more when I get another quart or so through it. Also guessing running a medium plug with the 25% might help with the temps and power as well. Still waiting for the LC4's to show up in the mail.

Ran my savage-x today a bit, it has an lrp28 in it with 47/16 gearing. Man that thing is retardedly quick. I ordered a 17T bell for it today. The engine tops out so quickly in second gear, even when running in grass that I know it will pull taller gearing easily. I have an 18T, but figured I'd try a 17T first.

Until next weekend... should have my RRP gears for the revo and the new savage bell by then.
 

2revo1maxx

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The os 21tm has always been said to have more torque than the 3.3, but that's about it. 25% and a medium plug should not have any noticeable impact on temps.
 

olds97_lss

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Had a tough time installing the RRP trans gears. Of course, the pin that sheared off wasn't included and they didn't thread the hole for it due to using the stock pin/c-clip thing. Also, the FOC in the kit required the stock output shaft. My trans never had that... luckily, I found an old parts bag from one of my revos I bought used that had reverse and it had the original output shaft in it.

For those that are curious, this is the shaft the RRP gear doesn't fit on:
2015-1102-RRPFocOnTRXFocShaft.jpg


I made a two speed end gear pin out of a hardened steel pin that was too big to fit by putting it in a drill and dremeling it a bit. This is it before I cut it to length:
2015-1102-HomeadeRevoTransPrimaryPin.jpg


Then I found out that when the gear on the two speed shaft was installed (which came with shims to take up slop), didn't line up with the hole, without shims. It had a raised lip on it that I"m guessing was to use some of the "gap" I should have had the shims for, but after gently starting the pin into the hole/shaft, I spun the gear to find it was crushing the bearing on the back of the two speed gear.

The raised edge in the center if this:
2015-1102-HomeadeRevoTransPrimaryGearIHadToMod.jpg


I ground it flat with a dremel, then used a couple of the included shims and everything lined up and worked fine. Wish I had taken a before/after photo. Just to remind myself if for nothing else.

So, as per usual, something that should have taken at most, 30 minutes... took 2 hours. I thought integy was the only aftermarket company that mis-measured things and had fitment issues... The difference between integy and RRP though is that I know the gears will probably outlast the truck. I just hope the pin I made holds up for a while.

Now I have the trans all back together (also installed a new trans case since 2 of the 4 mounting holes were stripped), the LC4 plug showed up and now I hope the weather holds out and stays warm so I can really run this thing this weekend.
 

olds97_lss

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Well, ran two tanks through it, then the trans started to free spin. Seems I didn't install the c-clip thing on the pin I made properly, or the pin was too long not allowing it to really clamp in.

I ground about 1mm off the pin, compressed the clip a bit and re-installed it. Then put heat shrink on it. I'm just glad the pin didn't shear on me as that's what I thought happened. I opened the trans and the pin was just laying inside it with the clip.

2015-1107-RevoTransPin.jpg
 

2revo1maxx

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Thanks to you I've decided to put my t maxx away and use the 18tm on my 'ol reliable revo chassis from '08. I'll post it up when it starts taking shape.
 

olds97_lss

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What pipe do you have? The 18TM likes a bit more free flow pipe than the stock pipe. A single chamber with the stinger drilled out works pretty good. It seemed to really like the THS pipe I have on my revo.
 

olds97_lss

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Well, I've probably ran at least another 1000cc's through it since my last post. Today was the warmest day out at 62F and after getting the tune adjusted for the day, I ran 500 cc's through it nonstop. Was a pretty good bash session in the grass and pine needles. It's still very clear that it doesn't have the grunt of a LRP28, but it does do really well. Can easily pull the gearing to pick up speed in grass that is tall enough to drag on the lower arms and in the grass, an easy blip of the throttle sends the front end in the air.

Odds are, will be my last run with this engine this year. I'll hold out hope for a while before dropping the old 3.3 back in it and changing the gearing to suit that engine better.

Looking forward to next year now. Really enjoying the run time it has over the LRP28 revo. Also liking the quieter sound.
 

olds97_lss

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Current videos of the engine in my revo. Probably around 2 gallons total by now. I didn't run it a lot last year. The first vid was a 1000cc day. The second was a 500cc day. Lost the MIP cvd pin/coupler as I was getting near to finishing my first 500cc filler bottle.

16/36 gearing:

16/38 gearing:

I'm liking the 16/38 gearing better. Wasn't a huge change in ratio, but it made a noticeable difference with my trigger finger. Gets up to speed much quicker and gets to higher RPM's in 2nd gear more easily. I think I was just trying to stick with "stock" gearing since this engine was replacing a 3.3 and should have had more torque. I decided to not care and just gear it for how I want it instead.
 

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