moto guzzi trotter engine rebuild

since the trotter’s going to be my main rally squeeze for the landsquid rally later this month, i decided i should maybe tear into the engine a little just to have a look to make sure everything’s good.
i could get the engine to start up ok, but it wouldn’t idle and the crank seemed a little too hard to turn by hand. i figured worse case scenario the crank would be trashed and then i’d have to hack some other engine onto the frame. part of me thought it would be hilarious to graft a crazy big-boy kitted e50 or something into that little frame, but if the engine was ok, i wanted to just leave it alone and ride it stock.

first things, i wanted to check out the top end and make sure the piston and cylinder looked fine.




the piston’s in great shape, though there’s quite a bit of carbon. i figure it’s probably from the old school oil from forever ago that it probably ran last.

look at the tiny intake port, it’s so cute!

the exhaust is pretty tiny too, and also has some carbon to scrape out.




once i had the clyinder off and the piston off the crank, i could tell something wasn’t right in the crankcase. the crank would spin, but not easily at all. the connecting rod bearing is in great shape and there’s no excessive movement anywhere, so there’s some good news at least.
the trotter engine has the same flywheel taper as puch, and uses the same electronics, though this stuff’s a bit older than you see on most maxis. the points still have a lot of life and i was getting great spark from the coil, so i’m just going to replace the sparkplug wire and leave the rest alone for now.


if all else fails i’ll just stuff something else in here.

the right side crankcase half came off pretty easy, and i was able to see why the crank was kinda hard to turn right away; the main bearing has siezed and the crank was spinning in it.

fucker bearing rusted solid.
no big deal though, turns out this engine uses your standard 6203 main bearing, same as puch and a bunch of other guys.

also, can we take a second to just look at how weird this engine is? so weird.

last time i removed the clutch i noticed the bell doesn’t just come off and i didn’t have time then to explore it more. turns out under that top hat seal thingy there is a snap ring that holds it on the crank.

crank out, thing’s a weirdo.

the crank has a nice sized oil groove in the big end of the conrod, and really wide triangle spacer washers between the conrod and the lobes, allowing lots of oil to get up in there.


so i got this far planning to just replace the bearings, make a new gasket for the case halves and put it all back together, but i figured i might as well pull it all the way apart and replace all of the bearings and seals.
actually, i didn’t replace any of the roller bearings in the transmission as they were all in great shape so i just did all of the big bearings. i’m getting ahead of myself here.

before tearing into the transmission, i decided to make sure the crank was salvageable. since it spun in the seized bearing, the new bearing would slide right on. not having access to a new trotter crank, i decided to go the hillbilly rout and dimple the bearing surface.
i’ve done this before on vespa large frame clutches when they would spin the brass bushing and the customer was too cheap to replace anything, but i’ve not done this on a crank before.
but whatever, it’s a moped. plus i have a set of e50 cases sitting on a shelf just asking for an 80 kit.

i went around the whole crank and made a bunch of dimples with my center punch. a bunch of them.

i totally didn’t take any pictures of the new bearings on the crank, but they pressed on nice and tight.

since the crank is fine, i tore into the transmission.
the sprocket on this thing really needs replaced but i’ll never find a new one. oh well!


i kind of really sucked at getting pictures of this stage. whatever, if you ever end up with a trotter and need help rebuilding it just email me.

engine cases all stripped down. well, except the studs. they were kinda stuck and i didn’t have a good rease really to pull them so i left them.

most of the rest of the engine in a bucket.

so i grab all four of my old seals and the two big bearings from the transmission and hit up the internet to see what i can find.
i am able to find three of the four seals on amazon and get them on their way, but one of the crank seals is a weird size that is apparently impossible to find on u.s. soil. i forget what sizes everything was, and everything’s at the shop 80 miles from here so i’ll leave a comment on this post tomorrow with bearing and seal sizes on the off chance of anyone ever needing to know.

i find my weirdo seal on this awesome website out of great brittan called, not only do they find it, but they have it super cheap, and shipping was like $3 or something! i went ahead and ordered it, not knowing how long it would take to get here.

fast forward a week and my two transmission bearings arrive along with both my crank seals, one from some warehouse here in california, the other from engineersmate! that seal got here before my two transmission seals had even shipped!
with everything needed to put the crankcase half of the engine back together, i decide to go for it. also, i cleaned everything up a bit.

not only were they super cheap and super quick, engineersmate also has a super rad seal on the baggy! a seal! GET IT??? ok, that’s enough.



popped the crank in and bolted it all together. i don’t realize it yet, but i totally put the crank in upside down. even though i checked it before i put it together, i still managed to goof it up. i realize it later and pull it apart and put it back together proper.

my transmission seals still haven’t shipped at this point, so i check engineersmate again and, sure enough, they have them for a lot less than the ones on amazon. i cancelled my amazon order and gave those britts my money and i expect i’ll probably have them sometime this week.

in the meantime i decided to go ahead and rebuild the transmission. the seals go in from the outside, so i figured it’s better to put the thing together rather than just have a bunch of parts in a bucket.



TA-DAAA! again, total fail on getting step-by-step photos. whatever.

the carb needed a good cleaning and a rebuild, so i did that next. you know how to rebuild a sha i’m sure, so here’s it after. pretty!

decomp got tore down, lapped in and put back together. new copper gasket.

the cylinder cleaned up nice! so did the head, but no photos of that i guess.


stock, the piston covered almost a quarter of the port windows in the cylinder at bdc. had i more time, i would have gotten some alum plate and made a ship to get the cylinder up to where it should be, and then deck the cylinder down to get the compression back to normal. i just want to get this thing on the road right now, though, so i just made myself a thick base gasket. it helped a lot, but the piston still doesn’t totally clear the windows. it’s hard to see, but here are some pictures.

if this thing ends up being a reliable runner, i’ll pull the top end off and do some work to it.

oh hey, there’s the head all cleaned up!

reinstalled the electrics and the clutch junk. also loosely installed the sprocket and other bits that go on the outside so that i don’t lose anything. once the transmission seals show up, i’ll pull that stuff back off, slap those seals in and button everything back up for good.

then i’ll throw it back in the bike and do a compression test. if it’s lower than i’d like, i’ll pull the cylinder again and take some more meat off the top (so far i’ve only done a little, mostly to make sure the cylinder and head were square to each other). it’s a stock moped, though, so it’s not like i’m going for super high compression.

once the engine’s in and i’m happy, i’m going to ride the crap out of it and see what else it’s going to need before the rally. i have already replaced all the cables (less decomp, but i’ll do it when i put the engine back in) and both chains. wish i could do sprockets, but those are weird and i’m not going to be able to find them. i’m thinking maybe tires, but i don’t know if i have the cash right now. if i do end up doing tires, i’m going to rebuild the wheel bearings while i have the wheels off. i’m thinking about dropping the fork and doing the headset bearings too, just to do them.

i should also think about registering it before the rally maybe. hmm.
it should make a fun rally bike!
i hope it can blast stock ciaos.

oh, and here are those rear pulley pics i took forever ago and never posted! such a weird bike.




1 Response to “moto guzzi trotter engine rebuild”

  1. 1 terrydean Tue02|0707|2013 at 9:22 am

    crank bearings – 6203 x 2
    crank seals – 25x35x7 and 15x30x8 (couldn’t find in the states)
    transmission bearings – 35x62x9 and 30x55x9
    transmission seals – 35x47x7 and 42x56x7


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