Port chamfer size

[dkatt]

Hello everyone, This is the first 2 stroke engine I am completely rebuilding. I have plenty of experience on automotive 4 strokes and a half dozen top ends on 2 strokes. Right now I have 3 engines torn down and have done a search here but don't seem to be able to find the answers.
1) on a 1994 XP, 657 I have bored the cylinders .25mm over and I realize I need to chamfer the edges of the ports leading into the cylinders, but what is the ideal size radius? I have googled port chamfer and I found an article calling out a 30 degree .5mm x 1.5mm chamfer. I agree with the 30 degree but the size seems quite large. Wouldn't that basically change the port timing by going this large?
2) I have some Tribond 1211 for the case halves, should this be used for the cylinder base gasket as well or something like Ultrablack? Blue locktite on the case bolts or red (would rather not use red unless it is necessary.) Is it recommended to use something like "Locktite bearing bond" to lock the bearings to the case or not necessary? (case halves look great, not pounded in)
3) The previous owner must have had the engine replaced because the block is not a 657X but the cylinders and the intake and carbs are 657X. Can I match the ports of the block to the intake and get some of the performance back?

I think this is it for now, fantastic forum. I have been reading for about 4 months now and trying to absorb a ton of info! Been busy all winter replacing wear rings, pump bearings and seals, fuel lines, oil filters, all rubber water hoses, adding fuel filters, and scrubbing oily hulls out. Thanks, Don

 

[ski-d00]

1. That is way too much chamfer for that motor. 30-45 deg is fine and you really only need .5mm-.7mm on the horizontal port edges and even less on the vertical edges. Basically, all you are doing is breaking the corner edge so that it won't snag a ring and those ports aren't that large so it has never been an issue on that motor. The 951s have much bigger ports and chamfers need to be larger which will snag rings when it feels like it anyway.

2.Threebond 1211 is the sealant of choice for the cases. No sealant on the cylinder base gasket. Just check your squish and use the correct base gasket thickness, they come in thicknesses from .3mm to 1.0mm. I have some extras in most sizes if you need a specific one. Not sure if you can still get them from seadoo. Bearing bond is not necessary, the machining fit is very good from Rotax. The Germans and Austrians have excellent motor building skills with very tight tolerances. No red locktite anywhere, except for the flywheel mating taper shaft which has a slight potential of slipping and stripping the keyway. Blue everywhere is fine.

3. You can certainly port the intake and it will have a noticeable effect. This is one area that is not good from the factory and you will remove quite a bit of Al. The exhaust ports can also be matched but these are a little better from the factory. You have to be careful when mixing 657 and 657X parts, there are many differences including the cylinder heights.

I still have a few boxes of 657 and 657X engine parts(cases, cylinders, flywheels, etc.) but no good cranks. I have no use for these anymore so if you're interested I'd sell cheaply. Let me know if you need anything. I also have a 94 xp with rebuilt motor that I'd sell for $900 that I let my friend ride in his small pond.

 

[dkatt]

Thank you very much for the info. It appears that the previous owner bought a long block reman and ended up with 657X cylinders and heads but a 657 block and crank, so I have approx .100" step down from intake to block at the top and bottom of the port.
I will probably have more questions once I start assembling the 951
Thanks again, Don

 

[rockman2010]

Hi i own an 1999 787 gtx rfi
which angle and width i should chamfer the ports in my case ... and do i only chamfer the horizontal edges
thx