piston/cylinder tolerance question
08-15-2005, 10:11 PM
#1
Race Car
Thread Starter
piston/cylinder tolerance question
Can you run tolerance group 1 pistons in a tolerance 0 cylinder? I have a block coated with nikasil and I have not measured the cylinder bore to be sure, but I believe they built it to tolerance group 0 specs.
If it is tolerance group 0, are my current group 1 pistons OK?
If it is tolerance group 0, are my current group 1 pistons OK?
08-15-2005, 11:11 PM
#2
Rennlist Member
The spec is .008 to .032mm with a wear limit of .08mm.
Group 0 cylinders are 100mm on the dot according to the tech spec book.
Group 1 pistons are 99.983 to 99.997 according to the book. That means you could in theory end up with a clearance as small as .003, which is less than half the tightest permissible clearance.
Regardless of what the books may say, the only way to know for sure is to precision measure your pistons and bores. When you get cylinders redone, you normally bring the pistons to the machine shop and let them bore the cylinders to fit the pistons.
Having said all that, if this is the motor you had Nikasil coated (US Chrome NiCom?), then I imagine you would need to get the proper clearance (and piston compatibility) from them. My aftermarket pistons, for example, came with their own clearance spec. Did your coater confirm that stock 944 pistons will work in the hard bores? I thought the whole point of that coating was to clear the way to use aftermarket (JE, Weisco, etc.) pistons?
Group 0 cylinders are 100mm on the dot according to the tech spec book.
Group 1 pistons are 99.983 to 99.997 according to the book. That means you could in theory end up with a clearance as small as .003, which is less than half the tightest permissible clearance.
Regardless of what the books may say, the only way to know for sure is to precision measure your pistons and bores. When you get cylinders redone, you normally bring the pistons to the machine shop and let them bore the cylinders to fit the pistons.
Having said all that, if this is the motor you had Nikasil coated (US Chrome NiCom?), then I imagine you would need to get the proper clearance (and piston compatibility) from them. My aftermarket pistons, for example, came with their own clearance spec. Did your coater confirm that stock 944 pistons will work in the hard bores? I thought the whole point of that coating was to clear the way to use aftermarket (JE, Weisco, etc.) pistons?
08-15-2005, 11:58 PM
#3
Race Car
Thread Starter
the shop doing the nikasil was told we'd be using stock 944 turbo pistons. they assured me everything would be fine assuming i was able to find rings designed for nikasil. 964 piston are the correct size and designed for nikasil. i am going to try and get the exact dimensions from the shop tomorrow to confirm if stock pistons will work, if they don't have the numbers i can get the bores measured. if they come out to be tolerance 0 (100mm on the dot), would repeating the diamond hone process remove (displace?) enough material to be workable?
08-16-2005, 05:08 PM
#4
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Originally Posted by OriginalSterm
would repeating the diamond hone process remove (displace?) enough material to be workable?
08-16-2005, 05:39 PM
#5
Race Car
Thread Starter
i did speak with the shop today. he told me all the bores are "exactly" 100 mm. i told him that i had tolerance 1 pistons and his response was that it wouldn't be a problem since the pistons i have are used and not new, and therefore probably not at the high end of the tolerance band anyways. his best recommendation was to leave the block as is and measure the pistons for size and shape, the put the engine back together and not worry about it. so i will be measuring the pistons and putting this thing back together (hopefully).
FYI...
he told me they could put as much as 0.040" thick nikasil on the cylinders.
FYI...
he told me they could put as much as 0.040" thick nikasil on the cylinders.
08-16-2005, 11:31 PM
#6
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Have the pistons checked and rechecked for their current diameter. I wouldn't want to take a chance on the pistons being at the top end of the tolerance and giving you half the minimum Porsche specified. These engines run a tight tolerance to begin with and if cut the clearance to .003mm you essentially have an interference fit with absolutely no allowance for any errors. Why did Porsche put the tolerance groups on the pistons and bores? Most lilkely because they had too to keep engines from siezing up and still minimize the clearances to keep blowby to a minimum.
My stock group 1 Pistons at almost 90,000 miles were close to the middle of the tolerance group. Yours may be on the high side, they may be on the low side, until you measure them accurately you just don't know. I wouldn't take the chance.
My stock group 1 Pistons at almost 90,000 miles were close to the middle of the tolerance group. Yours may be on the high side, they may be on the low side, until you measure them accurately you just don't know. I wouldn't take the chance.
08-16-2005, 11:37 PM
#7
Race Car
Thread Starter
Originally Posted by dand86951
Have the pistons checked and rechecked for their current diameter. I wouldn't want to take a chance on the pistons being at the top end of the tolerance and giving you half the minimum Porsche specified. These engines run a tight tolerance to begin with and if cut the clearance to .003mm you essentially have an interference fit with absolutely no allowance for any errors. Why did Porsche put the tolerance groups on the pistons and bores? Most lilkely because they had too to keep engines from siezing up and still minimize the clearances to keep blowby to a minimum.
My stock group 1 Pistons at almost 90,000 miles were close to the middle of the tolerance group. Yours may be on the high side, they may be on the low side, until you measure them accurately you just don't know. I wouldn't take the chance.
My stock group 1 Pistons at almost 90,000 miles were close to the middle of the tolerance group. Yours may be on the high side, they may be on the low side, until you measure them accurately you just don't know. I wouldn't take the chance.
