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Clubroom => Tech Talk => Topic started by: oldyzman on February 02, 2011, 04:39:20 pm

Title: Exhaust bridge port on cr250 83 Holeis in piston?
Post by: oldyzman on February 02, 2011, 04:39:20 pm

Gday gents,
I have just rebored my cylinder, the new wossner piston has no oil holes in it, should i drill a couple?
Also should the port be relieved back past the bore as it seemed to have been previous.
Thanks for any help...
Brett

Title: Re: Exhaust bridge port on cr250 83 Holeis in piston?
Post by: Freakshow on February 02, 2011, 04:53:24 pm

id say YEs to the above.

IF its been rebored then the bridge is now closer gap same as if it had power vavles poking in.  and that were it will head up and expand.

IF its air cooled i rip a row of holes or at least divots on the piston.  Does the piston have oil drag lines already milled in it ? if so id just run a row of holes on the bridge side, the ribs should cover the other sides.

Title: Re: Exhaust bridge port on cr250 83 Holeis in piston?
Post by: oldyzman on February 02, 2011, 05:04:30 pm

Thanks kerry,
So i put the piston and cyl on draw with texta the bridge onto the piston at tdc. then where do i drill the holes on the vertical axis,as the horizonatl one is obvious.
Thanks again
By the way i didnt send it today as i could only find big boxes.. will do 2moro.
Brett

Title: Re: Exhaust bridge port on cr250 83 Holeis in piston?
Post by: Mick D on February 02, 2011, 05:19:27 pm

(http://i677.photobucket.com/albums/vv132/mc125mick/FORUM163.jpg?t=1296627526)

Title: Re: Exhaust bridge port on cr250 83 Holeis in piston?
Post by: Mick D on February 02, 2011, 05:30:09 pm

Hi Brett,

Vertically, positioning of your first one as high as the thickness of the crown inside will allow. The next two roughly 12mm-15mm apart. As long as you position the holes so the bridge always covers them. On the outside of the piston I always twirl a much larger drill bit in holes to ”Just” remove the Burr from the hole edges. Make sure you have removed the inside burr as well(I use a pencil burr, dremel sort of thing), so it doesn't come free latter and become "foreign body" for a "score"

Yes definite chamfer and radius all ports, especially to create lead in for piston ring on any ports it passes.

Title: Re: Exhaust bridge port on cr250 83 Holeis in piston?
Post by: oldyzman on February 02, 2011, 10:47:57 pm

Thanks Mick and Freaky, Will do!
Cheers Brett

Title: Re: Exhaust bridge port on cr250 83 Holeis in piston?
Post by: Freakshow on February 03, 2011, 12:04:21 am

what mick said - good pic that. releive the bridge, but dont fork with the edges to much, you can over chamber edges and then they suck the rings in - common mistake on home port jobs

Title: Re: Exhaust bridge port on cr250 83 Holeis in piston?
Post by: oldyzman on February 03, 2011, 06:02:03 pm

I think i know what you mean Freaky. i borrowed a little pneumatic belt sander thats approx 8mm wide belt, i reckon i have machined of about .25mm or 10 thou from the bridge surface face. will that be enough???
Brett

Title: Re: Exhaust bridge port on cr250 83 Holeis in piston?
Post by: Lozza on February 03, 2011, 07:04:45 pm

I don't know how it's possible to over chamfer an edge? Ports eats rings because of the width and shape more than a chamfer.

Title: Re: Exhaust bridge port on cr250 83 Holeis in piston?
Post by: oldyzman on February 03, 2011, 07:55:18 pm

I think freaky is saying that if you chamfer too much off the vertical edge then it will make the port wider and eat the rings...
Brett