changing capacity to 501

[Bedfordowner]

Hi all. Does anyone know if it is possible to bore out the cylinders to take 70 mm pistons or would you need to change heads , barrel and pistons. My current engine is a standard 478. Also, if i did change all the top end can the original cases be used. Thanks for your help

[norbert]

It´s not that easy!
The 501 cylinders have a 71mm bore and Nikasil-cylinder liners that require special pistons (less clearance) and rings. You cannot combine for example iron cast liners with rings for Nikasil cylinders.

The 501 cylinder does not enter in the original 478 cm case. So you have to bore the case. Another option is to turn down the liners of the 501 cilinder, so they enter in the case. I have a W500 - 6 gear motor tuned with 501 cilinders like that.
Generally it´s not possible to combine 501 cylinders or pistons with a 478 ccm cylinder or piston, because the 478 ccm cylinders are about 5 mm higher and in consequence the bore of the piston pin is at a diferent height.

The 501 heads have bigger valves, but of course you can use 478 ccm heads as well (it will not be the same, as well as the cams are not the same).

One problem will be to find 501 cylinders and pistons, they are not easy to find.

I do have a pair of new old stock 71 mm pistons without rings for iron cast cylinders. They have been made especially for the Desert Ralley Camels (Paris-Dakar etc.) that have been used by Valentini & Co. I would sell them, but not sure about all the shipping/custom problems

The left one 478ccm, 69 mm diamter, 268 gr, with rings
The right one Paris-Dakar piston, 71 mm diameter, 261 gr without rings, for “478” ccm cylinder

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[Bedfordowner]

Norbert, thank you so much for your fantastic reply. This is the information i was needing. I do have the possibility to buy a rebuild 507 engine but it is expensive and a large financial commitment so would need to sell some other parts first as my bike budget is not great since retiring. I would like to build one last race bike based on the Valentini fully faired 350 which i have all the bodywork for, minus the screen if anyone has one, and was trying to decide whether a 478 engine with some careful tuning would do the job or go to the larger engine. Looks like more research before committing either way but i will bear in mind about the pistons that you have. Thanks again for your help.

[simonnorthroad]

Skidmarx in Dorset will be able to sell you a screen for sure. Meadspeed too I hear have lots of patterns

[Bedfordowner]

Hi Simon, it was good to talk to you recently. I did look at skidmarks before but then illness, etc stopped everything. Thank you for reminding me about them. Looks like the full engine will be easier so need to sell some bike stuff to help towards it. Regards, Keith

['It must be a .....']

Omega used to do 500 pistons to order including 70mm for the standard cast liners, giving 500cc
500 heads can also be opened up to take 28mm PHBH carbs
Like Norbert I found that 500 to 501 isn't just a simple top end swap and for me not worth the angst
Good luck with the project, Ian

[norbert]

My tuned 500W has 501 barrels, 501 heads, a L5 cam and of course PHBH 28. But as said, these parts are dificult to find and not cheap!
This motor will end in what probably will be my last project: a "pensioners bike " with heated grips, electric starter (my fist one ever) and after more than 45 years with clip-ons with a Brevetatto handlebar to rise it up a bit every year

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Without doubt the 501 engine (if posible with L5 (II)) cam is the best series motor, followed by the one from the 400 Dart and then the 350 Nikasil motors with "M" cam (X2, X3, Excalibur 350, K2(do they have M cam?). The cheapest way would be one of these 350ies maybe with 500 headsand 26/28 Dellortos

[Bedfordowner]

Thank for the details. Is this a 501 engine complete or one with a 350 bottom end? Also, love the wire wheels and discs. They look ideal but dare say difficult to get.

[norbert]

No, as said above, it´s a 500 (478ccm) 6 gear bottom end with 501 cilinder an heads plus a L5 cam. The rims are brandnew ones from Morad/Spain bought 15 years ago.