Motor rebuild project 86 piston in 87/9 long rod motor

[SBarclay]

Hey I'm working on freshening my top end going with an 86 68.5mm piston in my 87/9 motor. Going to add spacer plate so piston clears head and port timing stays the same. Any ideas or concerns would be great guys. Just wanting to make sure I'm not missing something.

[Grande huevos]

Anybody have any idea or tips on what info or measurements he would need to have to give to his builder ? Pretty sure he's looking To get his compression around 200-210. I know he runs vp 110 so that part isn't an issue..  I know there are some gurus lurking out there so any advice would be appreciated

[broken1]

Adding the spacer is to keep the 85-86 piston skirt from hitting the crank. If you have an 87-89 cylinder why not just stick with the 87-89 piston?

[SBarclay]

Adding the spacer is to keep the 85-86 piston skirt from hitting the crank. If you have an 87-89 cylinder why not just stick with the 87-89 piston?

 I guess I didn't mention my cylinder is already bored to 68mm which is the largest piston I could find for 87/9. I have damage on inside of cylinder from melting down the piston so now I need it bored. I have heard of others doing this as a cheaper option.

[SBarclay]

So I put everything together and my squish came out to .040". Calculated compression next and got 18:1 which is crazy. I have a 68.5 bore and 72mm.  15cc in combustion chamber plus .5cc would equal 15.5cc.
265 + 15.5 = 280.5 divide that by 15.5 = 18.09

does anyone have advice on what to do?

[supernutt]

that dome is way too small.  I think a 20cc dome with the stock .063 head gasket puts you right at 210psi so I can only imagine what a 15.5cc dome would do!!

[SBarclay]

that dome is way too small.  I think a 20cc dome with the stock .063 head gasket puts you right at 210psi so I can only imagine what a 15.5cc dome would do!!

so your saying I shoud up the size of the dome to lower compression down? 20cc would put me at 14:1

[mx250r91]

Is that 15cc measurement the flat plate volume of the combustion chamber in the head or the trapped volume above the piston measured at TDC on the assembled engine?

[Grande huevos]

Is that 15cc measurement the flat plate volume of the combustion chamber in the head or the trapped volume above the piston measured at TDC on the assembled engine?

Not sure if I'm following your question but the motor was assembled head was bolted down and we held the flywheel to keep the piston at TDC then added the oil..... If it was really pushing 18:1 compression would you even be able to kick it over?

[mx250r91]

Not sure if I'm following your question but the motor was assembled head was bolted down and we held the flywheel to keep the piston at TDC then added the oil..... If it was really pushing 18:1 compression would you even be able to kick it over?

Good that is your trapped volume, not the actual volume of the dome in the head (probably ~18cc dome). Yes you would be able to kick it over, remember a two stroke doesn't start compressing until the rings close off the exhaust port. Depending on the exhaust port height I would estimate your Corrected Compression Ratio (CCR) to be somewhere between 10:1 and 11:1 and statically crank around 220-240 psi, obviously use race gas only.

Here is a good read if you haven't seen it already: http://homes.ottcommunications.com/~red/uccr.html

[Grande huevos]

Good that is your trapped volume, not the actual volume of the dome in the head (probably ~18cc dome). Yes you would be able to kick it over, remember a two stroke doesn't start compressing until the rings close off the exhaust port. Depending on the exhaust port height I would estimate your Corrected Compression Ratio (CCR) to be somewhere between 10:1 and 11:1 and statically crank around 220-240 psi, obviously use race gas only.

Here is a good read if you haven't seen it already: http://homes.ottcommunications.com/~red/uccr.html

So how would you go about figuring out what cc the dome it is?? It is an oem head but we're not 100% sure that it hasn't been milled at some point... We have already started the bike and done 2 heat cycles the way it is and it started right up and purred like a kitten. Didn't sound funny no leaks nothing ....  We just want to be sure that it's safe to run and it's going to be reliable as can be

[mx250r91]

You would take the head off, put a spark plug in, place a flat piece of lexan with a small hole in it across the gasket face of the head and fill the head using a burette to measure the volume. Sounds like you did a good job putting it together and it is running good. Did you perform a static compression test on it yet for reference?

[Pumashine]

A builder runs the piston to top dead center. Fill the dome with tranny oil and let the piston down and catch the oil. Measure the cc to see what you have. Much easier said than done.

[udontknowme]

why would someone do it that way when you could measure it as you put it in :tongue-new:. then suck it back out with the same instrument you put it in with. never heard of someone trying to catch the oil but whatever works i guess

[Grande huevos]

A builder runs the piston to top dead center. Fill the dome with tranny oil and let the piston down and catch the oil. Measure the cc to see what you have. Much easier said than done.

we performed a uccr test with the motor assembled had the piston TDC then added oil and measured. It came out to 15cc plus 1/2cc for the spark plug so that's why we were thinking its a 15cc dome... That seems small but I don't know what a oem head has machined into it.  If I've read everything correct you don't want any more then 14:1 comp ratio, in order to get there he would have to buy a new head with 20cc dome