Porting old Maico's

[vmxservice]

I have a 1981!-250 reedvalve Maico. I want to bore it up with a 77 mm piston. Borexstroke will be 77x70=326cc. What would I have to change when it comes to porting. I mean, bigger, higher lower, wider, angle etc. I want a nice smooth engine wich will be used in sand tracks only.
Or if I take a stroke of 83, it will be a 400. Can I take the ports of a piston port engine?
I already sent an e-mail to Eric Gorr after reading his site. But I thought I would ask it here too
Thanks!!!!!!!!!

 

[EricGorr]

Overboring the cylinder nearly 11 mm will dramtically change the available space for the transfer ports. Is the sleeve thick enough? It may be geometricly impossible to do what you're looking for. I think that 73mm piston from LA Sleeve for either a KDX240 or a KX300 would be your best choice. Both pistons use a 18mm pin but have different timing heights. However I don't have any specs on the air cooled reedvalve Maicos of that vintage.

 

[vmxservice]

Hello Eric, thanks for your reply. The cilinders as such are, between a 250 and 400 almost identical except for the bore and height. I will make a new liner when fitting the 77mm piston and keep the 250 stroke. If I take the 400 stroke, then I would have to add 4mm to the cilinder base.
I was hoping you could give me some general tips on the porting of a cilinder like degrees opening and closing and the caracter that follows. I have read your website and found it of great information also. But I can understand that your expirience lies in the modern bikes.
Thanks again!!

 

[EricGorr]

vmx,
If you supply this basic information I can give you some port height and width numbers.
Bore
Stroke
Connecting rod length (center to center)
Deck Height (piston outer edge to TDC)

I have all the stock port configurations of an 81 Maico 250 on file.
I'll get back to you with the specs.
Eric

 

[vmxservice]

Hi Eric, that would be great!! Actually the cilinder I have is a pre-production or "works", if you will. It is from 1980 and was meant for factory riders. The only difference is that the '81 engine has a diffent distance between mounting studs. I have an official '81 cilinder as well so I can compair them before I do any porting.
The Measurements you asked for are:
Regarding 326cc: bore 77mm, stroke: 70mm, rod 131.7mm(400piston, 250stroke)
The full 400cc: bore 77mm, stroke: 83mm, rod: 137.2mm(400piston and stroke)
Deck height: the L-type ring should be 0.5 beneath the liner edge.
I think the easiest thing would be to insert the 400 liner with a 400 stroke. Also in combination with my personal weight of 90kgs, nothing beats cubic inches, right? I have a new liner of a piston port 400 available. Or would a piston port differ too much? I would have to add 2.5 mm, to lift the cilinder.
Besides this project I am also working on adding a reed to a 1981 490. This in combination with wiseco YZ490 pistons and Yamaha rod in an aluminium liner with nicasil. Any tips here to look at?
Again thank you very much for you help, Jeroen-Holland