It’s possible that the firing order is as you say, but on a modern flat twin you would have even firing impulses. Consider 2 revolutions in degrees, left hand cylinder fires at 0, 720, etc, right hand at 360, 1080 etc.

As for balancing, if you do nothing the two rod and piston assemblies balance each other except that they are longitudinal displaced, so there is a small rocking couple about a vertical axis. This can be reduced by rotating balance weights, one as shown in the photo, and the second being the flywheel with holes in it. Doing this will introduce a rocking couple about a horizontal axis half way between the central balance weight and the flywheel. At a guess the weights are chosen to make the residual vertical axis couple equal to the horizontal axis couple.

< gulp >

You are clearly preserving automotive history, Dave

Very wise to get this right, methinks !

MichaelG.

.

Ref. [for other innocents]
https://cars.bonhams.com/auction/15346/lot/306/1903-panhard-levassor-type-a-7hp-twin-cylinder-rear-entrance-tonneau-engine-no-5887/

From a very different perspective … I might be tempted to have a chat with:

https://www.dynamicenginebalancing.co.uk

A test-run on a dynamic balancing machine should reveal a lot.

MichaelG.

Just realised I’m making the assumption that the cylinders are horizontally opposed like a BMW bike. If they are side to side then the firing is uneven, Honda 250 was like that.

Comments on balancing stay the same

Well I’m about to make your day then, this link1 confirms that BMW bikes have boxer engines, and this link2 shows that in a boxer engine the pistons move in opposite directions. The second link looks very like a Beemer engine to me

On 13 March 2026 at 11:55 David Senior Said:

The cylinders are side by side, not horizontally opposed.

<p style=”text-align: left;”>In that case definitely uneven firing order, but the little weight and holes in flywheel are for balancing as I said before</p>

There must be Panhard Levasser enthusiasts out there who know the exact answer to your question.

Don’t risk catastrophe by using guesswork, either your own or some random forum speculator.

As Duncan says, a primary balance weight introduces a secondary force at 90 degrees to the original, ( Hence the liking for 90 degree twins, fours, and V8s, where the secondaries from one cylinder balance out those from another cylinder. Although this introduces a horizontal rocking couple)

A three cylinder vertical is naturally out of balance, but a vertical six (Effectively two 3s back to back) is smother because one end of the engine balances the other, although not entirely free of couples.

The firing impulses will be asymmetric for a side by side two cylinder four stroke, as will the induction and exhaust pulses.

“Boxer” engines are smoother because the primary out of balance forces from one cylinder are balanced by those from the opposite cylinder. But as the number of cylinders increases, the greater the tendency for horizontal couples causing the engine to “squirm” about a vertical axis.

Steam locomotives, if balanced for  horizontal forces (The reciprocating components) produced vertical force – hammer blow) which the civil engineers hated because of the damage done to the track.

One of the southern railway chief engineers avoided this, by not using balance weights, but gave passengers in the front carriages a very rough ride by jolting them back and forward, in time with the piston thrusts.

It is possible that the flywheel was drilled, deliberately to throw it off balance, to negate the out of balance forces from the balance weight on the crankshaft. (Some 3 cylinder engines  have weights on the crank pulley and out of balance flywheels in an effort to produced better balance and smoother running). In which case the the parts must be fitted back in the original position. (Often by having an offset bolt hole, rather than a dowel).

Howard

I have just looked at another variation in the current Royal Enfield Interceptor crank, 270degrees, with a front mounted balance shaft to reduce the vibration.

https://www.bike-parts-royalenfield.com/royalenfield-motorcycle/650-moto/INTERCEPTOR/INTERCEPTOR_650_TWIN_EURO_4/CRANKSHAFT/10/27/0/10