I got another thought and want to ask you about it. I have seen that Axel has a 650 cylinder kit to convert the 500 engine only by reboring the engine block and without the need of the spacer anymore. I was thinking that besides the carburator are there any other major differencies between the two engines that has to do with power output? I am not sure about the head differencies maybe 126 has bigger valves.

The valve sizes of a 126 (650) are:--33m inlet and 28mm exhaust, as against the 500D which are 32mm inlet and 28mm exhaust---I wouldn't bother to change them. If you are going to fit Axels 652cc barrels and pistons, when the machine shop are opening up the crank-case (and GO NO FURTHER than 652cc with a 500 crank-case), get them to lightly skim the head (to ensure absolute flatness). If you fit a 0.5mm copper head gasket, this combined with the extra engine capacity will up the compression ratio a fair bit, without spending masses of money. 'Port and flow' the cylinder head, 35/75/75/35 cam and a decent exhaust will result in much more rapid car. You will probably have to obtain a Weber 28 IMB carb and re-jet it----probably 1 up on the main jet and 1 up on the idle jet.

Well the difference in valve sizes is not worth to mention. As the Hobbler mention, 650 barrels a 35/75 cam, ported head and ofcourse a bigger carby, maybe 30mm plus the lighter 500 flywheel sounds like a quiet fast engine, maybe faster than a stock 650. As for the compression i think i can control it with a proper gasket. I might ask Axel if he has any feedback on this mod.
What i like to know is why the older 650 cylinder kits had to sit on a 1cm flange that now with those new cylinders is not necessary.:yum:
You know i enjoy the 18hp with my family in the car, we are doing 70-75 km/h and we enjoy it but this until we meet an uphill and i can ensure you that we have a lot here:bang: The 70 can become 30 and this is not good.

gordinir8 said:

What i like to know is why the older 650 cylinder kits had to sit on a 1cm flange that now with those new cylinders is not necessary..

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The crankcase is 1cms higher on the 126 engine so the standard 650 cylinders are 1cms shorter and need that spacer plate. I only recently discovered that those longer cylinders are available to convert the 500 engine and I think they are a great way to go.

When fitting the 652cc Nardi kit at Radbourne Racing, we re-fitted the standard 500 head onto the 650 barrels. I honestly cannot remember what head gasket we fitted, but it must have been one compatible with the bigger barrels. e fitted the Nardi camshaft and the put it all back together--went like little rockets! The Nardi cam timing was:--35/60/80/35, with valves set at 0.25mm (I managed to retain one of the camshaft cards from when I was at Radbournes)

Toshi 975 said:

The crankcase is 1cms higher on the 126 engine so the standard 650 cylinders are 1cms shorter and need that spacer plate. I only recently discovered that those longer cylinders are available to convert the 500 engine and I think they are a great way to go.

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So the 650 crankcase is also different from the 500. Yes i think those new cylinders are cool, didn't like that much the adapter option.

Hi

My engine is 500 crankcase with 650 barrels

The machining of crankcase was done by previous owner

I bought a brand new 650 head from axel or fdricambi. It has bigger ports than the old 500 head. Also bigger exhaust elbows and pipe diameter of 126 exhaust is bigger. This then requires the correct exhaust brackets

See pics

https://flickr.com/photos/36060059@N06/sets/72157659892844069

Well as must have dining table!!! Nice and clean overhaul you got there thanks for sharing. Have you drive it as a 500 and now as a 650?

1500500 said:

It has bigger ports than the old 500 head. Also bigger exhaust elbows a

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Brilliant photos from a very meticulous rebuild.
I must check the port and elbow diameters because I didn't notice any difference when rebuilding myself. Presumably we are talking about just a few millimetres?
Another difference I remember is that the 500 has bronze valve-guides and the 650 has steel guides. I always assumed that bronze is a better material for this purpose?

gordinir8 said:

Well as must have dining table!!! Nice and clean overhaul you got there thanks for sharing. Have you drive it as a 500 and now as a 650?

Click to expand...

The whole car is in pieces for a respray so I haven't started it yet. Hopefully by the end of the year...

So i talked with Axel Gerstl shop and a very nice gentleman (those guys there now how to communicate with customers) told me that if i use those new 650cc cylinders on the 500 block the compression ratio will be higher than the normal 126. If i leave the rest original i will get about 23 hp. If i add a sportscam, sportexhaust and a Weber 28 + bigger jet i will get about 25-28 hp, maybe more if i make modifications to the head and get the engine balanced.
I guess that someone can overcome the compression problem by installing a thicker head gasket.
Sounds like a good and promising option but more expensive than installing a 126 engine/gearbox combo which can be found for less of the price of this cylinder kit.

But this would give you a new engine except for crank bearings......something the old 126 engine would not be.
Go for higher compression. The engine can handle it......stronger bang.......faster engine!

Yes but also old gearbox, you also have to buy new carburator. Total cost will be even more than overhauling an 126 engine. It has pros and cons.

Sharing my personal experience with 500-block, 650 cylinders/pistons, factory 500cc cyl head, and factory Weber 24 carb.

I fully appreciate that this thread is several years old so I don’t expect any responses....but if I get some, all the better.

I have a 69’ 500 block/crankcase with the aforementioned 650 cyl/pistons with spacers installed and have just recently started the engine for the first time following a 7-year storage period.

Having fully forgotten what I did when I built this engine it’s taken me a while to piece together what I have in/on it but....so far I can confirm the standard 500 crankcase/block (thermal coated for heat dispersion), and the aftermarket 650 pistons and cylinders. I can also confirm that I have the original 4240095 cylinder head for the 500cc. I also have the original Weber 24 carburetor on there....which to my surprise has apparently been running very rich for the motor (the plugs were covered in carbon soot).

Out of curiosity, I tested my compression and repeatedly came up with 170-180 psi on both cylinders. Testing was conducted with both spark plugs removed and with the carburetor held wide open. Yet despite this high compression ..... i have no memory of the car running poorly or with any detonation.

I plan to test drive the car in the next month or two depending on weather (we’ve had exceedingly warm temperatures here in Washington DC for this time of year). Assuming the test and tune drives go well.....I’m planning on attempting to install a small supercharger on the engine. I suspect that with my high compression and factory heads and carb,....I’ll be asking for all kinds of headaches and upgrades but.....”even the mighty oak, was once a little nut that just held its ground”.

Very clean and tidy engine---well done. Interesting approach to the 'finish'--never seen (or thought of) having the crankcase and head thermal coated. Your valve-spring tool in very interesting; I have a similar one but could never seem to get it work correctly (I am aware that it is the type used for taking off/re-fitting valve springs with the head still in place). Interesting that the carb, despite being technically too small, actually seems to be running rich, although, Fiat fitted the '24' on the 594cc engine in the 500R to slow the model down. With the '28' carb on it, due to the cars lighter weight, the 500R was quicker than the Mk1 '126' (with the same engine) which was replacing the 500.