Why do you need the pistons BEFORE you bore?

[retroicon]

Hey all,

Alright, this makes no sense to me ...

If I'm buying a brand new set of pistons at, let's say, 81.5mm ... why would the shop need the pistons in order to bore the cylinder? Wouldn't a new set of pistons at a specific size be something pretty simple to do? I mean, shouldn't they be the right size, give or take a thousandth or so?

- G

 

[Matt_E]

Because the individual piston sizes can vary a bit. The cylinder bores need to be clearanced to about 3-5 thousandths of an inch to the piston, so even a little bit of variation is a bad thing.

 

[Crab]

and its nice when they are sold in matched sets.

 

[Mark Gomez]

It also means and hints that the shop or person who is going to bore your cylinder knows what they are doing. :reporter:

 

[nightrider05]

It also means and hints that the shop or person who is going to bore your cylinder knows what they are doing. :reporter:

Exactly what I was going to say.

That is one thing I ask when looking for a shop to do engine work. We have a couple places around here that failed on that one. I ended up sending my cylinders to John Zigler. Would rather pay to ship the parts than have some hack locally mess them up.

 

[wsuwrhr]

Hey all,

Alright, this makes no sense to me ...

If I'm buying a brand new set of pistons at, let's say, 81.5mm ... why would the shop need the pistons in order to bore the cylinder? Wouldn't a new set of pistons at a specific size be something pretty simple to do? I mean, shouldn't they be the right size, give or take a thousandth or so?

- G

"Give or take a thousandth or so" could mean the difference between a rattler or a siezer.

Either way, many people would blame the machinist.

Brian

 

[wsuwrhr]

It also means and hints that the shop or person who is going to bore your cylinder knows what they are doing. :reporter:

....and then there is that.

 

[Big Kahuna]

"Give or take a thousandth or so" could mean the difference between a rattler or a siezer.

Either way, many people would blame the machinist.

Brian

Years ago I had to do a quick bore. Ran to Birmingham to pick up pistons, stopped at a machine shop (Guy that owned it raced ski's), picked them up an hour or so later. Ran it for a few months. Right before a race I checked compression, really low, like 150. I was running a non girdled ADA Head at the time. Should have been much higher. Sent the cylinders off to get bored. Guy called me and told me I had nothing to worry about.......... As far as seizures go. I had about 17 thousandths clearance. Just a tad loose.

 

[wsuwrhr]

That motor musta made HUGE noise.

Brian

Years ago I had to do a quick bore. Ran to Birmingham to pick up pistons, stopped at a machine shop (Guy that owned it raced ski's), picked them up an hour or so later. Ran it for a few months. Right before a race I checked compression, really low, like 150. I was running a non girdled ADA Head at the time. Should have been much higher. Sent the cylinders off to get bored. Guy called me and told me I had nothing to worry about.......... As far as seizures go. I had about 17 thousandths clearance. Just a tad loose.

 

[Matt_E]

Meh....I had one that was 20thou clearance. Didn't really make huge noise, but compression was super low.

Some jackass sold me a "big bore" cylinder with matching pistons (84.00mm). The clearances were so big that I had to bore up to 85.
My fault for not checking, but still.

 

[retroicon]

Alright, I guess I won't argue because I'm not an engine specialist or anything ... just seems odd when I've seen guys ( including me ) run a cylinder with major scoring for years without an issue, and I'm sure the factory just slaps two pistons in from some big case full of pistons sitting along the assembly line.

Are you saying that the factory is picking two pistons, then boring each cylinder individually based on that particular set ... I can't believe that would be true.

- G

 

[dbredleader]

Piston bores / fitment

I have always had to provide the pistons prior to boring... I'm sure the OEM process uses standards that are fine for stock setups. Quality control on Yamaha & Kawasaki are minimal. Just enough to be dependable. LOL ( If all the sleeves are bored with the same tooling, all the bores will be the same size )

 

[ATS_Scott]

Alright, I guess I won't argue because I'm not an engine specialist or anything ... just seems odd when I've seen guys ( including me ) run a cylinder with major scoring for years without an issue, and I'm sure the factory just slaps two pistons in from some big case full of pistons sitting along the assembly line.

Are you saying that the factory is picking two pistons, then boring each cylinder individually based on that particular set ... I can't believe that would be true.

- G

In manufacturing things like pistons there is a little thing called tolerance. That means every single part is slightly different depending on the required tolerance. It impossible to get every part identical without huge additional cost. So cylinders are not Bored per set, but individually. If you assemble them in the wrong hole you might get one loose, and one tight. Instead of both pretty close.

 

[DAG]

i always see if a machine shop would bore w/o the pistons,, if they will i'll call someone else. its a great test.

The shop knows what they are doing if they insist to have the pistons before the to bore.

 

[Boris]

Are you saying that the factory is picking two pistons, then boring each cylinder individually based on that particular set ... I can't believe that would be true.

- G

No, the factory stamps the final bore on the cylinder and an appropriate piston set is installed at assembly. If final bore is out of a specified range, the part goes back or is scrapped.

 

[Matt_E]

I'm sure the factory just slaps two pistons in from some big case full of pistons sitting along the assembly line.

Are you saying that the factory is picking two pistons, then boring each cylinder individually based on that particular set ... I can't believe that would be true.

- G

Each cylinder is stamped from the factory with identifying marks to match them up the the right pistons.
They are not just "slapped in there".

 

[#ZERO]

You should supply both piston kits including the rings to any machine shop doing the cylinder boring and include information about your complete setup to help determine the proper clearances.

 

[WFO Speedracer]

One thing you are not taking into consideration here is the shops measuring equipment, even if you gave them the size numbers off top of your new pistons that does not mean that if their 85.05mm and your 85.05mm would be the same, this is especially true in places with large variances in temperature from night to day, all these factors come into play when doing cylinder boring.

Anyone who can bore and finish a set of cylinders within an hour , I don't want doing my cylinders. It is best to let the cylinders cool down and recheck your CLEARANCE measurements. If you have the pistons in hand , you check the pistons with your mic, bore them and finish hone them , then recheck your clearance, then there is not a problem down the road.

 

[kevbo]

One thing you are not taking into consideration here is the shops measuring equipment, even if you gave them the size numbers off top of your new pistons that does not mean that if their 85.05mm and your 85.05mm would be the same, this is especially true in places with large variances in temperature from night to day, all these factors come into play when doing cylinder boring.

Anyone who can bore and finish a set of cylinders within an hour , I don't want doing my cylinders. It is best to let the cylinders cool down and recheck your CLEARANCE measurements. If you have the pistons in hand , you check the pistons with your mic, bore them and finish hone them , then recheck your clearance, then there is not a problem down the road.

Its nice to make them perfect but, our light weight cylinders are not the most dimensionally stable nor are the running temperatures consistent enough to benefit much from ultra precise machining. Most jetski bores must have excessive clearance to survive the horrible treatment they get from their owners.