GUIDE: Pro tips to building a Puch Polini


There seems to be a lot of nervousness when it comes to building these kits. There’s not much too it, but there are some Polini tips I’ve picked up from Devin at Motion Left Mopeds, Peter, the MA community and a few little things I’ve figured out along the way. Most all of this stuff exists in a thread or blog already, but I thought it’d be nice to compile it in one place.

Case Matching: Make sure you cut the top transfers from the Polini ALL the way back into the crankcase. This is going to maximize flow though those transfers and keep your cylinder/piston cooler. Here are some photos of my case. Looking back I could have gone a little deeper towards the top of the case (making each trough a little deeper), feel free to.

polini case match 3
polini case match 1

Cast Matching 2.0 A couple years after the initial case match I took the cases further. I did an in depth blog post on it here. Some highlights.

Brass sleeve inserts for a perfect case match and cylinder alignment.
brass inserts for proper port/piston alignment

Deeper top transfer grooves.

Deeper and smoother transfers.

Intake Porting: A quick word on Polini porting: Don’t start porting if you have no idea what you’re doing. Build simple, learn, then progress. That said, if you’re ready, this is where things get fun.
Hookay, If you look at the intake on your cylinder you’ll notice the intake/boost ports are bigger on the cylinder wall than up by where the reed block rests. You can get some power by opening these up. Word of caution: don’t go nuts on these if you’re running a 2 petal setup. A Polini’s torque comes from some wicked intake velocity. If you open these transfers too much you’ll hurt yourself. So how much is just enough? When I initially built my bike I took each port back 2.5mm. Recently I had my motor apart to prep and port it for racing this summer and took it back another 2.5mm.

This is from the latest porting I did, so the ports are already taken back 2.5mm and have another 2.5mm marked.

polini porting
polini porting

You want to keep a smooth ramp maintained, so make sure to take your grinding all the way right up next to the cylinder wall like this.

polini porting

Max Intake Porting: This is what I took my intake too in prep for the 24TM and Dio reedblock. You can read a whole post on it here.

Exhaust Port Chamfering: Fairly often when Polini’s seize it’s because the piston catches the top edge of the exhaust port and chips. Smooth that leading edge of your exhaust port out to help. Also, see the tip on oil a little ways down, it’s helps with this too.

Smoothing out the Cylinder Intake Mating Surface: Polini’s come from the factory with a bumpy intake mating surface. You don’t want this.

polini porting after

So bust out whatever smoothing tool you prefer and smooth that buddy out. No leaks!

polini porting

Intake Tightening: An issue some people have had with Polini’s is warping intakes. Sambo made this really helpful video illustrating it.

Basically, the intake bolts extend too far and when they’re over torqued, pull up on the cylinder wall causing it to warp. There’s a few things to do about this. First thing is to check your cylinder to see if you have this issue, not everyone does. Take your piston ring off, tighten down your intake and reedblock and see if the piston drops freely through. If it does, you’re fine. If it doesn’t, shorten your bolts up a little and try again.

When you tighten your intake/reedblock down for good (which you’ll do after installing the cylinder, see below tip) you want to make a gasket sandwich. Spread a real thin layer of RTV onto both sides of both the cylinder-reedblock gasket and reedblock-intake gasket. This will allow you to use less torque and still get an airtight seal.

Lining up the Piston/Cylinder: You’ll notice there’s some rotational play in your cylinder on the engine studs. Because of the precise boost/intake transfers in the cylinder and piston it’s important to get these lined up properly. By leaving your intake and reedblock off you can see through the intake to the piston (see below photo). Once everything is properly lined up go ahead and tighten up your head bolts.

little things

Ring End Gap: It’s always a good idea to check your ring gap. I set mine to .011. This was a little conservative, but I still have loads of compression and I wanted to be safe. Do as you please, but at least check it.
Phil from the 77 team went more in depth on ring gaps in a great post on the 77 blog.

The Ideal Setup: Before you naysay me on this, let me clarify that this is just my personal favorite daily blaster Polini setup and what I feel to be the easiest setup to build super reliable. Stick with the 2 petal and go with a 15 Bing. The cheap pipe of choice for this setup is the Estoril. Modify it (take out the interior baffle) for more top end. Until last fall, I ran the 15 Bing/Estoril setup with 16×40 gearing for a super torquey, unbreakable 50-52mph bike. I’ve run that bike at WOT for over 20 miles straight with no problems. Now you can mix and match for different flavors. Want more torque? Get a Proma Circuit. Want something shiny? Get a chrome Boss. Have a little more money to spend and want the best pipe hands down for a 15 Bing Polini? Get a Motion Left Mopeds REV-RUN.

Oil: Another one that everyone has a different opinion on. I’m convinced Amsoil Saber is the best stuff out there. It’s pretty cheap (~$10 a quart) and is designed to run at 100:1. ’77 had a great blog post a while back on Polini’s and why they seized. They talked to Polini USA and the dudes at Polini said they were running too much oil and suggested switching to 60:1, because synthetic oil burn hotter than gas. So what better oil to run in your Polini than one that calls for even less oil (and thus less heat) than 60:1. I broke my Polini in at 70:1 and run it at around 80-90:1. Brennen bandit even (accidentally) ran his Grande at 200:1 for two full seasons with no problems. I’m not saying you have to run Saber, I just don’t see why you wouldn’t.

Breaking It In: Another debatable topic, but here’s the advice I got and what worked great for me. Make sure to take it easy (stay off the pipe) in the first 10 miles. After that feel free to get on it a little. I was ripping full throttle on mine after 20 miles and never looked back. Feel free to take a more conservative or more aggressive route, that’s what worked great for me though.

Heads: Lots of options out there that work for heads, but STAY AWAY from aftermarket 50cc hi-comp heads. When I initially built my bike it was with the 50cc hi-hi comp head Benji sells packaged with the Polini and my bike was not happy with it. Lots of pinging and hesitation at high rpms even with 93 octane gas. I switched to the stock puch hi-torque head (one that matches the Polini fin pattern) and it solved all those problems, and had better midrange and top end. It’s what I run on every Polini. Graham (aka Moped Factory) from the Cranks makes custom cut heads specifically for Polini’s if you want to try those out.

custom puch polini head

Extra go fast things. These aren’t neccesary to have a rippin Polini, but for the sake of getting all this information in one place here it is. Don’t go hacking into your cylinder if you don’t know what your doing, I’m not responsible for your shit blowing up.

Exhaust Porting: Polini’s are real sensitive to exhaust porting. Being a race cylinder, they’re already real aggressive…but if you want to push the envelope you can take it bigger. Stock Polini exhaust size is 29.5mm wide (thanks Tom!), which is 67% of the cylinder bore. There are some different numbers out there as to how wide you can go and still be safe. I’ve heard between 70-72%. I decided to widen mine ~.75mm on each side taking it to 31mm, about 70.5% of the bore. I also raised my exhaust about .5mm. I did a whole post on this if you want to check it out. It ended up looking like this,
race ready

Reed Mod To maximize flow on my 2 petal, I thinned out the bridge. Doppler makes a reed block that has no bridge, but Devin ran it and had the reeds sucked into the intake, so I didn’t cut it out completely. Here’s a before and after.
UPDATE: This ended up being a pain. The reeds flowed too much in the mid range for the taper of the 15 bing needle and made it a nightmare to tune. I could have tried out some different needles or chucked the needle into a drill and hand shaped it, but in the end it was easier and safer to just switch back to an unmodified reed plate.

The Next Level (Feb 2011)

Over the winter I decided I was going to start down the path of taking scary busey to the max for a Polini e50. Seize big or go home. So I snagged up motomatics radical Polini Maximizer and matching dio reed block. It’ll be matched up with the TM24 I’ve had lying around. With that a few things need to be done…

Prepping For the Dio Reed Block: Because the dio reed block is so wide, you have to lose the two big side cooling fins on top of the cylinder. I don’t believe you have to remove the front ones, unlike with the malossi and polini 4 petal blocks. The cylinder I’m using for this already had them gone though, so I can’t confirm.
cooling fins are for sissys

Awesome Polini Posts: I’m going keep a collection of Polini threads/blog posts that have some good knowledge and opinions from some of the masters. Feel free to send me some you think should be listed here.
Cuperzack did a rad blog post on some next level polini stuff:
MA post about a blown polini piston, turned into an interesting discussion:

So that’s it so far, in no way is this comprehensive or absolute so please feel free to add stuff. I’m also not the Polini expert. There are some people out there doing way gnarlier stuff with this kit than me. I just wanted to compile some of the stuff that’s helped me run this kit successfully in hopes that more people feel confident building these, and that they’ll do it right. I’ll keep this updated as things progress.


18 Responses to “GUIDE: Pro tips to building a Puch Polini”

  1. I’m glad you finally posted this! I look forward to going through this thoroughly once I start final assembly on my Mini Mag build. Nice work!

  2. remember, the polini piston is DESIGNED for a 50cc head, not a 65cc one. All graham needs to do for a polini head is set the squish by shaving it down to the right clearance.

  3. 3 conrad

    Well done!

    I agree about breaking in, I started blasting early and haven’t had any problems.

    Interesting about the 50cc hi head, I’m gonna have to give a stock head a try now.

  4. 4 zack

    my “break-in” with my last piston/ring involved installing it on my derbi, popping it in neutral, and seeing how high it reved up. Once content, I took it to the races. No need to baby these.

    • 5 gabeb

      hahaha love it. i had a feeling, but hadn’t done that myself yet. i should’ve added that i destroyed my head gasket (like 4 diff pieces when i took my head off) and was running with a MASSIVE headleak for 15-20 miles inside my first 50 miles, no problems.

  5. 6 zack

    I’ve blown a headgasket or two with mine (last one a year ago in LA after blasting 65+). Its very disappointing when nothing else fails except that. I’m the air leak master, but never any real lasting damage.

  6. 7 Graham

    thanks for the props

    notice any difference from the head, or did you change too many things at once?

    sounds like you guys had a wicked time. wish i could have been there.

  7. 8 andrew


    what do you use to smooth that intake to cylinder bit? File? Dremel?

    Seems like you would want to clean it up but keep it perfectly flat.

    • 9 gabeb

      all the porting work is done with a dremel. i used the cone shaped high speed aluminum cutting bit for the intake work.

  8. 10 Matt

    Hey how exactly do you chamfer the exaust port, with sand paper?

  9. 13 Matt

    Would you happen to have any pictures of the chamfering when it was done

  10. 14 Charlie

    heyy thats my tread! wow i feel special. This blog is where i started my dream of polini blastin.. thanks man. it’d be nice if you added a line about timing in here tho. i believe that was my problem, causing my seize. i saw in a different post that you dremeled out your cdi plate to get better timing, how many degrees were you at with the bing on there? also any timing changes when you do the intake porting? i put mine full advanced as it came thinking that it would be enough. i really shoulda checked it.. ehh too anxious to ride i suppose. costin me a shiny pennie, or more like $60 for a new piston.

  11. 15 conrad

    Do have any results from the TM24 and dio reed block yet? I’m really curious about the results.

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