70cc Ø46mm kit with MVT S-RACE air-cooled cylinder head for MBK 51 / Motobecane AV10

Warning, not road legal, for competition use only.

This is undoubtedly the most powerful kit on the market for MBK 51 air-cooled 70cc!

THE CYLINDER ASSEMBLY, THE STORY

Here is the MVT S RACE 70cc kit for MBK 51. Following numerous requests, MVT wanted to release a beautiful large bore cylinder. They therefore took their knowledge base from their AGP (ALAIN GODOU PRODUCTION) / & MVT brands on 50cc competition cylinders to produce a true 70cc competition cylinder!

Unlike other 70cc cylinders on the market, this isn't a gentle setup that only delivers its power at 8,000 rpm; you'll see that this is clearly something that demands to scream and rev!

*Is it necessary to point out that various MVT cylinders have won in competition, including the base that served as the little brother of this one?

We are therefore dealing with a cylinder of 46mm diameter, which is therefore 69.61cc on original stroke 41.8mm and 74.51cc on long stroke 44.9mm RRD (note that fitting on long stroke requires major modifications).

The side port transfers are split, meaning the channels have a partition at the inlet but not at the outlet; there's only one side port on each side! This design has proven successful on various 50cc engines. We have two standard rear transfer ports. So we have a 6-port split system and a 4-port intake. Note that the separation between the exhaust port and the side intake transfer port is very thin. We won't be modifying or altering that area! It's already at its maximum.

The transfer ports at the base are huge and they look magnificent; you can immediately see that you're on a racing cylinder... Which is made to breathe!

Regarding the exhaust mounting, there is a simple M32mm thread, no possibility of mounting a 2-screw flange ball joint from PEUGEOT 103.

The finish is a bit disappointing, especially the transfers; it's not great for "Instagrammer" photos and impressing your online audience. For those who remember, back in the '90s, ZETTA, MALOSSI , and POLINI cylinders weren't finished this well. We've become much more discerning! And it won't affect the overall performance, but clearly, the finish "could be better"! Then again, a simple bead blasting over the deburring would instantly give it a different look. So let's be lenient. This cylinder has other strengths!

The nicasil coating appears to be of superb quality, and the honing is also a parameter found on the new MVT cylinders!

Hallelujah, three base gaskets are included in this cylinder; some brands should take note! Especially at such a low price, as it will cost around €200 including VAT with a cylinder head (2024 price)! That's the price of a complete kit without a cylinder head from MALOSSI or POLINI these days.

THE PISTON

MVT has decided to supply its piston with a 12mm pin instead of the 13mm pin originally fitted to MBKs. You will therefore need a slightly thicker 12mm pin cage, so a 12x16x16 cage, and given the power this beast can produce, it's a very good idea from MVT !

So we're dealing with a 45.95mm diameter piston MVT . To keep prices down, MVT doesn't provide wear specifications! When the assembly has too much play, you either replace the entire assembly or find a piston that fits!

In terms of length, the piston is 47 mm, which is more than enough to accommodate a long-stroke 44.9 mm piston by blocking the exhaust port at bottom dead center (BDC). The piston weighs a hefty 80 grams, so it's not exactly lightweight! For those looking for a different piston, the compression stroke is 22 mm.

The piston ring is sold separately and is 1mm thick to ensure optimal high-end performance. It's a high-quality, pre-run-in steel ring! The lug is located at the rear center. The ring end gap must be strictly adhered to; this is a bridged cylinder with air cooling! Don't mess around with it!!! I recommend a ring end gap between 0.22 and 0.25 mm.

DIAGRAMS

MVT set the exhaust port timing at 186° and the intake at around 128°. With its massive bridged exhaust port and huge intake ports, along with its large transfer ports at the base, it's immediately clear what we're dealing with! We can't guarantee the accuracy of the port timing diagrams, given engine movement, displacement, squish, etc., but we can't be far off the mark!

This is clearly superior to all 70cc moped cylinders on the market; it will require a large carburetor, an exhaust that works and matches a powerful engine, therefore, an MVT SP2, an ignition that keeps up, a variator that exploits 10,500 rpm!

If we want to install it on a long 44.9mm stroke, obviously, we'll end up with completely enormous port timings! The only solution in this case is to reduce the base clearance and machine the cylinder head, or even reshape the cylinder head to achieve acceptable values, especially on the intake side!

We could tune the beast for even higher diagrams, but honestly, on an air-cooled engine... It's perfect as it is!

THE CYLINDER HEAD

The cylinder head is truly magnificent, with a slightly recessed chamber to protect the excellent Viton O-ring and prevent flame damage. It features a hemispherical chamber, a polished finish, and a very nice squish zone.

Polygonal cylinder head, oversized to provide better overall cooling, it's really great!

Regarding the compression ratio, I made sure to take the actual measurement for you, as I hadn't had time to do it on the PEUGEOT 103. The result is 13.6:1… with a squish clearance of 0.8mm.

Competition demands it: The spark plug thread is a long base thread, you can fit good racing spark plugs with good heat transfer between the cylinder head and the spark plug!

CONCLUSION

Is it a good kit? Absolutely, it's an excellent kit. If you're looking for leisurely rides in grandpa mode, and you don't have any mechanical knowledge or the budget for a good exhaust, ignition, large carburetor, and quality crankshaft, forget it... Move along, it's not for you.

If you want a kit that really rips for having a blast on the track and on very sporty rides, and you have some mechanical knowledge to properly assemble and use everything: go for it!

This is a kit that requires installation with a large carburetor and a super-reinforced, high-quality crankshaft; otherwise, you'll break it. In our opinion, it must be installed with a crankcase that has good measurements, such as the POLINI crankcase. A premium ignition is also necessary. You'll need to be able to exploit the power of 10,000-11,000 RPM, so obviously, the only exhaust that will work with this setup is the MVT SP2! Use 98 octane fuel with a high-octane additive, and 3% Motul 800 Road oil.

*Remember that on an air-cooled cylinder with a bridged piston, the cylinder head gap is extremely important; 0.22 or even 0.25 mm seems to be a minimum. For the squish, another important parameter, 0.8 mm seems like a good target!

As an option, we offer a Stage 1 preparation. This preparation involves modifying the intake and exhaust ports by increasing the port timing and improving the flow, thus increasing the cylinder's performance. An improved piston and skirt are also included in this modification. Please allow a little extra time for this preparation!

As an option, STAGE 2 preparation is available! This upgrade increases the exhaust port timing to 188° and the intake port timing to 127°, making it a truly demonic cylinder! Upon specific request, an adaptation for RRD long-stroke crankshafts can be made for the same price!