Bores seem tight on 998 +40
Turbo Mini Forums

Had my block rebored +40.. Just started to put everything back together, the crank torqued down nicely with new bearings and thrust washer but my Pistons seem a little tight on the bore, so much so that there is metal coming off in the oil.
I would appreciate any advice or feedback.
Hopefully the picture shows my point.

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What pistons are they and did you give them to the machine shop to measure for the rebore?

Time to pull it apart, reclean, inspect and test. If it is down to the bores, pass straight back to whoever did it.

Is that pitting on the cam shaft or the photo?

when you re built did you check each piston moves nicely on the crank and in each bore?

They are 20773 Pistons. Yeh they were given to the machinist, they all move in the bores but there is a little more resistance than I would expect. Also the metal on metal (piston on bore) as seen in the pic makes me think I'm not over reacting! Iv spoke to the machinist, I'm going to leave it all built to show him then take it from there...
The cam is brand new.

Check the clearance with a feeler gauge between the piston and bore. I would expect there to be around 5 thou 0.12mm on the skirt. I would be amased if it was the piston clearance it's more likely to be a bearing nipping up or cap on the wrong way round.

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1/4mile in 13.2sec @ 111 terminal on 15psi

Fresh bore new rings, its just taking the peaks off the hone.

From the photo, looking at the bore on the right (where the piston is up out of view) it doesn't look like they have made any attempt to remove the burr - created by machining so quite normal - from the bottom of the bore.

To me, that photo clearly shows a ragged edge.
Have you tried running your finger around it, that is a very easy (although potentially painfull) test.

Clearances could be fine (new bores/rings are always tight) but that ragged edge just looks all wrong.

Schrödinger's cat - so which one am I ???

yep my block came back from the shop with a super sharp burr on the crank end of the bores i found this while cleaning the block so i took a dermal and small stone to it creating a tinny angle back it dose t need much just enough that theirs is not a 90 degree where the piston can rock and shave its self down this can be done wit a file as well

if there is marks on the piston than ill put my money on the skirt catching on the lower bore the shop should have taken a file to in and put a tinny angle on it..

if there is any marks on the piston or bore id ask them to sort it as you have payed for a quality service not a hack this is similar to what i went threw with mine i ended up with a refund and then went to a shop with a good reputation.. less cash and a better job the first time!!!

all said this is just my opinion form little knowledge take what you will talk to an expert