A bit of taking apart and investigating has revealed that I have a 650cc engine conversion and not the standard 500 I thought. Bad news is the pistons are worn and out of tolerance.

Due to this I am looking to replace but wondering if it is better to get a cylinder and piston kit. However does anyone know if these normally come honed?

Thanks for any help.

You would surely expect the barrels to be ready to install??

Apriliarider said:

Due to this I am looking to replace but wondering if it is better to get a cylinder and piston kit. However does anyone know if these normally come honed?

Thanks for any help.

Click to expand...

I can assure you that they do come in the honed condition. I just checked as I have a couple of new sets of pistons and barrels here. Should you be interested let me know as I am located quite near to junction 18 of the M25 and could possibly save you a few pennies

Further to this I'm having a few issues.

Does anybody have the outer diameter of the cylinder of a 594cc kit?

Apriliarider said:

Further to this I'm having a few issues.

Does anybody have the outer diameter of the cylinder of a 594cc kit?

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Don't quote me on this or use the dimensions for anything other than a rough check because I only have a pair of cheap vernier calipers, but 81.45 mm give or take a millimetre.
You can see on the image that the outer is stepped twice but I don't know why.
Are your new ones too big or too small?

MAL_5340 by Peter Thompson, on Flickr

The issue I have is that my crankcase bore I have is 82mm exactly (pretty much, again done with verniers so not perfectly accurate).I appear to have some funny cylinders which are 650cc but with an 82mm external diameter and don't require any spacer between the crank and barrel hence the strange size crankcase bore.

A pair of cylinders I have got as replacement 650cc are fine in diameter lower down but then there is a step meaning they are slightly too large (0.45mm diameter too large) and would either need some taking off or the crankcases boring out slightly.

Was wondering if the 594cc would be a good fit but concerned the tolerance would be too great (0.5mm too much, as I understand the 594cc external diameter is 81.5mm). Do you know if this would be the case?

Seeing as we are both being somewhat less than precise in our measurements I think that the speculation is not going to be useful. The best approach would be to take the crankcase and new barrels to an engineering shop and have them measure them. They will understand the exact tolerance needed to allow the components to slip together.
The problem may be because the crankcase will have been bored out to fit the exact dimensions of the original 650 barrels that someone has obtained; the aftemarket components may not always be exactly to original spec.
Ultimately, if it gets too complex, a straightforward way out if you are happy to go back to 499cc., would be to get hold of another used, 500 crankcase, which I suspect may be relatively easy to do.....or go back to Plan A and get oversized pistons and get the barrels rebored.

When I worked at "Radbourne Racing" the 650 barrels we fitted were supplied by "NARDI", and I seem to remember that they were modified VW barrels (1/2 of 1,300cc---650cc!). We did NOT fit any spacer plates under the barrels. I also cannot remember there being any 'lip' at the bottom of the barrel. We often got the cars in brand new, stripped the engine out and down to a bare crank-case and sent the crank-case away to be machined whilst we carried out any suspension mods that were required. This way, the car retained its original engine number. I wonder if the original barrels that you have are modified VW barrels? Peter's suggestion is very sound--take the barrels that you wish to fit, along with the stripped down crankcase, and ask their opinion as to how much more the apertures need to be machined out. The only way to ascertain if, and what thickness, you require a bracing plate is to fit the crank, rods and pistons all together when you get the crankcase back from the machine shop and do some measuring.

Right, after much deliberation and re-measuring (and some extremely helpful advice from Toshi as well), I have decided the best way to go is for an oversize rebore and hone.

Having a slight problem in that I can find 77.2 oversize pistons and a 77.4 oversize piston and ring kit, but no rings for a .2 oversize. Does this mean that .2 oversize will run with standard 650 rings or should I be getting the .4 oversize kit?

Thanks for any help.

As to my cylinders they appear to have NAVA stamped on them. Have uploaded a picture that hopefully helps see this and also one showing the overall shape of cylinder. These do not require a spacer, anyone seen them before?

Why not go direct to a Piston-ring manufacturer? I can think of 3 off the top of my head:----"F.W Thornton", "Piston Rings UK" and "H.P.Rings". maybe one of these companies can help you out.

These are 650 barrels that do not need spacer plates and are made to upgrade a 110F engine to 650cc. They are available from a number of vendors. Regular 650 barrels are shorter than the 110F engines 500cc barrels. In the attached picture the 650 standard barrel is on the left and a standard 500cc barrel from a 110F engine is on the right.

John