Measuring for rebore

I am in the process of getting my '87 430 auto rebored to 3rd oversize - I have a Wossner piston on the way at present.
I would like to know the best place to measure the bore so that clearance is correct. The manual says to measure at the very bottom of the bore but I have been told to go up a little - 1/2 to 1 " up from the bottom. I want to get it right so I am asking those who know what is best for my engine.

I am under the impression that Wossner write the clearance on their packing. Should I use that or can somebody recommend something different.

Many thanks
Rowan

Probably not much help answering what you ask.
The bore should be cylindrical while the piston is tapered.
There were no instructions pertaining to clearance and/or type of use intended with the Woosner piston I got but that was a few years ago.
What I find troubling from a design aspect is that the head gasket is designed for the stock bore and sticks out into the cylinder as the bore is increased.

Take it all to the machine shop that will be doing the boring so they can properly do the measurements. Yes , Wossner includes the clearance that they desire. A good machinist will want the new piston and the cylinder to make it right.

Bottom line.....any machine shop that will bore without the new piston in hand does not know what they are doing

Yes - of course I have already had the bore measured by the ONLY good 2 stroke machinist in town ( that's how I know it needs a re-bore ). Many engine shops can't do 2 strokes properly. I am just waiting for the piston kit to give to him so he can do the job.
My main concern is where to measure the clearance - at the very bottom of the piston skirt or up a little bit. I have been given conflicting advice.

Thanks

Clearance is measured a specified distance up from the edge from the bottom. Most piston manufacturers supply that fitment point along with the clearance required.

Exactly can't bore till the piston is in hand to measure it. I can rough bore till the cylinder is cleaned up so the proper sized piston can be ordered. Then and then only can the cylinder be finished bore to size with the proper clearances. I rough bore when the cylinder is scored really bad.

Did you replace the crank bearings and seals?

The engine man measured the bore and found that it is slightly over the 87.0mm current size (about .002") - hence the 3rd oversize piston is on the way. He has told me he always works with 'new pistons in hand' and he will not touch my cylinder until it arrives.

The engine has done only about 25 hours on the current bearings - but I will check them for movement anyway.

Thanks

then why do you need to measure it?
a correct job is done the way others have mentioned above..
i give the shop a list of available piston sizes..the shop bores until round and the cylinder is cleaned up, then lets you know what size you need. after i get the piston for them, they then hone to correct tolerance. there is no measuring by the customer, as you need bore gauges to get a correct measurement. this involves me going to the shop twice to drop off the jug, then the piston but it insures a good job.
any wossner i have bought has the tolerance they want on the product label.

What he said. Just give the machinist the piston and let him do it.

You guys are right it is on the label. At least the clearance value. In theory (my theory) the bore should be exactly on the millimeter or half a millimeter. I will have to examine a Mahle box and some brand x ones. There does not seem to be any information on the packaging for the Cagiva era replacement.

With my Auto being air cooled I tend to go to .10mm over rather than the .08mm stated by Wossner. Only because the Auto's run hotter. If it's a watercooler then I would reckon it's best with the recommended clearance. Those with L/C Auto's probably will give you the correct answer.

I am not sure they run hotter in most situations seem to coast but if an air cooled bike was used in winter conditions, snow with some depth it is easy to reason the cylinder would most likely be colder than a water cooled one.

Note that piston says 84-88. The earlier 420/430 pistons have the ring slot a bit farther down. Some other differences I have noted in the past.

A two stroke bore wears the most in the middle where there is the least material.
A finished bore should measure the same all the way through or it's a balls up.

Justintendo. I don't have any bore measuring gauges myself so that is why I had the machinist do the measuring. He told me it is worn and he recommended re-boring rather than just putting in a new piston of the current size.

My original question was regarding where to set up the clearance - at the very bottom of the skirt/cylinder or slightly up the bore.

Once I finish boring and honing the cylinder I wash it. Then port it. Wash it again the burnish a moly paste into the cylinder walls and piston skirt. This eliminates the first start up wear. I pump some two stroke mix into the lube hole on the right ignition side bearing. (Pre start up lube.) the crank seals is lubed when there installed.

The moly treatment works awesome. We tested it on a seasons worth of running hard. At the end of the season we took the engine apart to find the cylinder and piston still looks like we just assembled it.

I was taught in engine building to measure the piston opposite the wrist pin holes. But I check it at the top about 3/8" down. The wrist pin holes and near the bottom. About 1/2" up. The upper half of the piston expands more it's closer to the heat in a air cooled engine. It also depends on how hard the engine is pushed too. If she's run hard and put away wet lol, I put the clearance a tad larger. With the bore I check for taper. I allow .0000" only top to bottom. The worst taper I have seen is .00005". But 99.99% it's .00000".

The widest part at the "Bottom" front to back.

I learned that in 2 minutes doing a search on this forum.

Dukkman is the most correct. Folks boring the cylinders are not all the same. It should be correct.

Nothing is better than a dial bore gauge and it worth the investment.

Their even more things to check. I found a couple bores that failed on a Maico and one Husky I had bored were not parallel
Bore guy did not even set cylinder square ! Out of round, taper , out of tolerance etc. are others.

I now double check each cylinder done. top , middle , bottom and a least four sides. If ports are not in way check the cylinder bridges areas
I have seen these messed up on the way the cutter hits them.

In hot climates bore it to the high side for clearance on the big bores. Give a bit more room to expand.

In my experience today the Mahle of yesterday seems to be one of the best pistons. They are lasting longer than the new ones.

Rowan , I have used HPE in Christchurch and always had a good service .

I have a dial bore plus a inside, outside mics too. With the dial bore you also need the master gauge ring to set and make sure the dial bore is set correctly.