Tomos A3 bottom end with A35 top end. Trim piston skirt?

Hi all,

I'm going to be putting my bigger carb/exhaust (thanks Treats!) on my 1978 Tomos A3 Bullet that somebody put an A35 top end on. I've heard the A3 Bullets were able to have lower/higher performance models by just changing the length of the piston skirt and trimming them makes them all essentially the faster model.

First, with an A35 top end is this even relevant to me? If it restricts the intake port, if the A35 has the intake in a different spot I'm not sure if it matters. FWIW the original owner started this A35 conversion but looks like they stopped as there was no intake or carb and the ignition wire was cut with no boot and no spark plug. I'm doubting they swapped the A3 piston out.

Is there any before/after pics? I'd like to compare it to mine but I can't find any pictures or a guide of how much or where to trim. If I do it I'm thinking an angle grinder followed by files and steel wool to clean it up?

Thanks in advance. I searched here and google and found nothing other than people saying they did it. I've always had fun working on things with wheels and engines but this is my first moped.

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

A3 has a 10mm wrist pin while the A35/A55 have a 12mm. I imagine they stopped when they realized it simply wasn't going to work with the crank in the A3?

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

I didn't pull the cylinder off yet to see if they swapped pistons or not. Both being 38mm would it work if they took the bottom end plus the conn rod, piston, etc from the A3 and put the A35 top end over it? No other mods? I'm told it's not uncommon of a combination but I don't know if there are any other required modifications that needed to be done to make it work.

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

If you use the a3 piston with the a35 cylinder you need to cut a window in the a3 piston. It will definitely work and work well when done right.

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

Edit: just found a post from 2016 stating an A3 piston will run in an A35 cylinder, but stated the piston windows need to be opened up to reap the benefits of the A35 top end. I'm guessing this is the same reasoning as with the A3 head where you trim the piston skirt to open up the intake?

Question still stands, are there any guides or photos of this trimming? I'd like to do it right and make sure I take the right amounts off in the right places. Thanks.

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

Thanks!

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

> Hugh Mann Wrote:

> -------------------------------------------------------

> Edit: just found a post from 2016 stating an A3 piston will run in an

> A35 cylinder, but stated the piston windows need to be opened up to reap

> the benefits of the A35 top end. I'm guessing this is the same

> reasoning as with the A3 head where you trim the piston skirt to open up

> the intake?

>

> Question still stands, are there any guides or photos of this trimming?

> I'd like to do it right and make sure I take the right amounts off in

> the right places. Thanks.

If you're running the a35 cylinder you don't need to trim the piston skirt because it's reed valve. Trimming the skirt is for using the a3 cylinder which is piston port and trimming the skirt changes intake timing which if done right would make it faster.

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

Makes sense. The old post I found stated:

"the A3 piston will actually run in an A35 cylinder, so if you want the reeds you just need cylinder, head (to make it look good but not necessary) reeds, intake, carb, and pop it right on the old piston. to really take advantage of the reeds you need to cut the windows into the piston."

I looked at the windows in an A35 piston and I'm not sure I could cut those nicely without an milling machine. I don't see it happening with an angle grinder or Dremel. Hopefully it's not too big of a problem without them.

Thanks.

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

> Hugh Mann Wrote:

> -------------------------------------------------------

> Makes sense. The old post I found stated:

>

> "the A3 piston will actually run in an A35 cylinder, so if you want the

> reeds you just need cylinder, head (to make it look good but not

> necessary) reeds, intake, carb, and pop it right on the old piston. to

> really take advantage of the reeds you need to cut the windows into the

> piston."

>

> I looked at the windows in an A35 piston and I'm not sure I could cut

> those nicely without an milling machine. I don't see it happening with

> an angle grinder or Dremel. Hopefully it's not too big of a problem

> without them.

>

> Thanks.

I usually draw the window, drill holes in the 4 corners then use a Dremel to finish it up. I've done it quite a few times. Don't be scared, get in there!

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

A few examples that I did with a drill and a Dremel. It's not hard just need a steady hand.

Screenshot_20200820-120915.png
Screenshot_20200820-121043.png
Screenshot_20200820-121148.png

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

Thanks for the photos, seems doable. Dumbing it down a bit, is there a special process to determining the location and size of the window? I'm guessing shine a light into the intake as I move the piston between TDC and BDC and try to ballpark where to cut so that at no point is the side of the piston blocking the intake?

Is there any other purpose as to cutting a window instead of just a slot without a bottom, aside from structural integrity of the piston? I wouldn't do it, but just to understand the intention of the window. I see that a stock A35 piston actually has 2 windows. (edited)

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

tomosports.jpg

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

Martin Fotak /

Currently running a35 cyl/head on an a3 crank/rest. Havent cut holes on the piston and it works great, pulls over 50kmh with half throttle(havent tried wot yet since its in a process of breaking in).

Just make sure that the bottom of the piston opens intake port all the way when i tdc.

My crank was made for 10mm pin and piston uses 12mm pin, so i had to use bronze bushing instead of needle bearing.

Folks On other forum(croatia forum which has a lot of tomos mopeds and users) claim that if you make holes in the piston it gonna make you slower since holes were designed to make sure mopeds dont go over 45kmh which is illegal (not sure about this claim). check out so called L piston in attached pic, i use it as i said without cutting any holes.

B9090B5B-9D5D-4F0C-8F4E-C569C4C670AE.jpeg

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

Kevin Bishop /

Press out that bushing and use a Honda needle bearing. They are 10mm. You won’t be sorry.

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

Martin Fotak /

My piston has 12mm pin and conrod is 14mm. I dont think such thin needle bearing exist.

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

Kevin Bishop /

A3 crank with Honda needle bearing and 10mm wrist pin/ A3 piston. Been running mine for years. Not a35 12pin wrist.

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

Question.

For those of you that just did a A35 cylinder swap and left your stock A3 piston and crank, why are you changing the wrist pin and bearing and not just leaving those stock ? Durability ?

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

Tweaker trash /

I put a puch piston in a a35 cylinder

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

the op just fell off the map, bronze is not as robust as needle bearing but some say the inner surface of the conrod is not prepped for a needle bearing.

Re: Tomos A3 bottom end with A35 top end. Trim piston skirt?

I've run the bushing plenty, some of those bikes were beat on for years, it works just fine.

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