Mismatched pistons

If must retain the mis-matched pistons ---

Just guessing, but seems the existing scenario might better match respective weights of 1/4 with 2/3, than would result from pairing both heavier pistons on 1/4 and both lighter pistons on 2/3.

For example, in the existing scenario, the combined weight of 1/4 at TDC equals the combined weight of 2/3 at TDC.

Otherwise -- if pairing both heavier pistons on 1/4 and both lighter pistons on 2/3 -- the weight of 1/4 at TDC would exceed the weight of 2/3 at TDC.

Am thinking it's preferable to have the combined weight of 1/4 pistons as they rise and fall together matching the combined weight of 2/3 pistons as they rise and fall together.



Good Fortune!

Dont mean to high jack a thread, but I pulled the motor today (always a pain by myself) but decided to go deeper and found one of the pressure plate screws had also been broken , and once again drilled off center so bad they installed a sheet metal screw with a washer as a clutch spring holder :whistle: . I'll pull the bottom case tomorrow and look for more gremlins.
Im actually surprised I got that motor idling smooth and running good before finding the leaks and going deeper. :laugh:

If I find 4 different pistons, I'm going to drop kick the neighbors cat.

sc/ant864 wrote: OldSchoolZ-It's funny you mention the broken bolts, I broke 3 of the main crank bolts and 1 of the bottom case bolts while preparing to split the cases. One hour later, after smacking each one with a pointed drift, drilling and an easy-out everything was kosher. I would also suggest heat if they wont budge, martin_csr suggested that for stubborn exhaust studs.

wireman-Measured the pistons with my crappy calipers and came up with 70mm.

OldSchoolZ wrote: Ive seen crazier shiz though. Thats proof many people have no business working on bikes. If someone is taking the time to pull a top end, they might as well do it right.(Im sure they got lucky regarding the weight issue) :whistle:
Thats ok, I have to pull the bottom case half off the motor in the bike I just bought cheap. They broke off 2 bolts on the lower case, JB welded crap and cross drilled the bolts trying to remove them. Time to EDM them I guess.
Some goof balls have no business working on anything.

This should be everyone's motto
"Lord grant me the bravery to fix things I know I can fix, the strength to accept the things I cant, and the wisdom to know the difference"

Actually it would have been an easy fix if they hadnt drilled off center in a recessed hole. Oh well, I'll get it straightened out pretty soon.

i've seen lots of guys use mismatched pistons racing hurt one and had other ones from another engine that got hurt and use them instead of buying a complete set happens alot as long as they are close to the same weight and same compression why not the crank and cylinder head won't know the difference as long as the clearances are right. if you have to remove weight take it from under the dome not off the skirts as those pistons are bigger at the bottom and you will have a tendency to make the piston to cylinder clearance looser than you want. in fact i have had to do it myself on one of my race engines it worked out fine ran that motor for 2 seasons. its not that big a deal how did the motor run with them in it.

Well actually it ran quite well...while it was running...which was a small feat itself. My main worry is that the problem I had with different compression numbers across the cylinders may have been because of the different skirts, since they will heat and cool differently. I don't know that it's a huge deal, but another friend of mine that raced MX said the differnt surface areas will cause that. On another note they seem to have grind marks at the oiling holes which makes me think they at least weight matched them and they are Weiscos. Also when I clean them and weigh should they be weighed with the wrist pins and circlips as a unit?

timebomb33 wrote: i've seen lots of guys use mismatched pistons racing hurt one and had other ones from another engine that got hurt and use them instead of buying a complete set happens alot as long as they are close to the same weight and same compression why not the crank and cylinder head won't know the difference as long as the clearances are right. if you have to remove weight take it from under the dome not off the skirts as those pistons are bigger at the bottom and you will have a tendency to make the piston to cylinder clearance looser than you want. in fact i have had to do it myself on one of my race engines it worked out fine ran that motor for 2 seasons. its not that big a deal how did the motor run with them in it.

I should elaborate that I meant heat in the way of a heat gun not putting a torch to your case, as I'm sure you know, just don't want someone stumbling onto this and putting a flame to their cases. Yeah now you'll have to Helicoil or thread-sert no? BTW what's EDM, a version of of the 2 mentioned methods of thread repair?

OldSchoolZ wrote:

sc/ant864 wrote: OldSchoolZ-It's funny you mention the broken bolts, I broke 3 of the main crank bolts and 1 of the bottom case bolts while preparing to split the cases. One hour later, after smacking each one with a pointed drift, drilling and an easy-out everything was kosher. I would also suggest heat if they wont budge, martin_csr suggested that for stubborn exhaust studs.

wireman-Measured the pistons with my crappy calipers and came up with 70mm.

OldSchoolZ wrote: Ive seen crazier shiz though. Thats proof many people have no business working on bikes. If someone is taking the time to pull a top end, they might as well do it right.(Im sure they got lucky regarding the weight issue) :whistle:
Thats ok, I have to pull the bottom case half off the motor in the bike I just bought cheap. They broke off 2 bolts on the lower case, JB welded crap and cross drilled the bolts trying to remove them. Time to EDM them I guess.
Some goof balls have no business working on anything.

This should be everyone's motto
"Lord grant me the bravery to fix things I know I can fix, the strength to accept the things I cant, and the wisdom to know the difference"

Actually it would have been an easy fix if they hadnt drilled off center in a recessed hole. Oh well, I'll get it straightened out pretty soon.

sc/ant864 wrote: Well actually it ran quite well...while it was running...which was a small feat itself. My main worry is that the problem I had with different compression numbers across the cylinders may have been because of the different skirts, since they will heat and cool differently. I don't know that it's a huge deal, but another friend of mine that raced MX said the differnt surface areas will cause that. On another note they seem to have grind marks at the oiling holes which makes me think they at least weight matched them and they are Weiscos. Also when I clean them and weigh should they be weighed with the wrist pins and circlips as a unit?

I would keep the wrist pins with the different pistons, as the Wiseco pins may have a different weight to them than OEM. You can check that though, and post up the results. Most would have a set of 4, either OEM or Wiseco, or whatever not mixed.

An EDM is an electic discharge machine. It burns the broken acrew out with high intensity electricity. It even leaves all your aluminum threads intact. It really is amazing, even removes it if a tap is,broken off in the hole.I first had one done when a cylinder stud broke and was frozen for 20 plus years. Check into it. Its about 40-50 dollars a hole, cheaper than cases in my situation. A few places do it in Houston.