piston skirt problem?

[elliottles1]

good day folks,
i am fitting a yz250 cylinder on my dt250 engine for extra fun. this is a straight swap apart from having to move the stud holes 2mm. bore and stroke etc. is all the same.
the whole top end will basically by yz250, however i was intending using a dt250 piston for added life, the only difference is that this piston has the rear skirt intact, however i find that with the extra width of the yz inlet port that the dt piston skirt appears to "catch" on the bottom of the inlet port on the way down.
has anyone had any experience with this type of thing. i always thought that the removal of the rear skirt for the race set up would be to increase induction, however could it  be that it avoids this "catching"?
any thoughts welcome, before i fire her up!
cheers
les.

[m in sc]

it shouldn't catch, but if you arent sure, shorten it a few mm.

[Dvsrd]

Is the port edge chamfered? If the cylinder has been rebored, all port edges need to be chamfered, to make life easier for piston and rings.
Also, a little reprofiling of the port, into a slight U shape could help too.

And of course, correct piston clearance is most important.

[elliottles1]

M in SC
ill do more work on it tomorrow, at the moment the rebore is on the tight side and it may be "pushing out" into the inlet port.
i have researched since posting and the IT250 barrels have pretty similar ports but with skirted pistons so it should work.
cheers
les.

[elliottles1]

Dvsrd,
hi again.
yes, just rebored and the ports are chamfered. it doesnt catch solid, more seems to "ride up" onto the bottom of the port. clearance is tight so i will hone slightly tomorrow and if that helps.

M in SC, are you saying that by taking a few mm off the bottom of the piston that it wont reach the bottom of the port?

cheers
les.

[m in sc]

it might be. you could also radius it, small arc on the bottom. I can see or feel it so hard to say.

[Dvsrd]

BTW, how much wider is the skirt, than the port?
Or put differently, is there sufficient overlap for guiding the piston past the port?

[elliottles1]

Dvsrd,
there is 5-6mm overlap to the side of the inlet, increasing as the skirt reaches the bottom of the port (as the port curves in).
les.

[Dvsrd]

Then I really do not see that it would be a problem. Filing/ sanding a 30 degree chamfer on the bottom end of the skirt may also be a sensible precaution. {In addition to chamfered port edges and correct piston clearance)

[elliottles1]

folks,
thanks for the feedback, feeling more confident that i wasnt putting the wrong piston in wrong barrel, i honed a bit more today, and am much happier with it now, there is also a good radius on the port and a chamfer on the piston skirt just to be sure.
i will post how the initial start up goes!
cheers
les.

[1976RD400C]

Here's what my RD Wiseco piston does banging away with a widened intake port and .004 clearance.
Edit : forgot to mention it never caused a problem

[elliottles1]

1976rd400c
thats exactly the type of thing that i am looking at.
when you say "banging away", do you mean that you can hear it loud?
mind you, 4 thou is a huge clearance!
cheers
les.

[1976RD400C]

1976rd400c
thats exactly the type of thing that i am looking at.
when you say "banging away", do you mean that you can hear it loud?
mind you, 4 thou is a huge clearance!
cheers
les.

No, I don't think that it makes any more noise than normal. I just noticed that lower edge of the skirt when I torn it down to change the pistons. When it was fresh it was .0025 and the piston wear got it to .004+ . My intake port has been widened to 46mm from 39mm. I replaced the pistons and the clearance was about .003 and have run it 2 years like that. I'm going to check again and I think a bore job will be needed. I said "banging away" because it sure looks like it must have been, but it runs great and doesn't make any more noise than a normal.

[elliottles1]

1976rd400c,
cheers for that, i am much happier now!
les

[m in sc]

id put a slight arch at the bottom of the piston skirt up to avoid that. but thats just me.