piston to cylinder wall clearance is supposed to be around .001 to .0015 inches; you have to use inside and outside micrometers to check it......"feeling" the clearance just does not _______ make the grade.....and I hope you have the volvo service manual on reconditioning that engine.....it will give you the specs and illustrate how to check the clearances.......and I also hope the machine shop had the new pistons so that they could properly finish-hone the cylinders to end up with the correct piston to cylinder clearance........if the piston to cyl clearance is more than .003 inches, you will experience piston slap, esp when cold........volvo designed those motors to have tight piston to cylinder wall clearances......for longevity as well as power......if you put it together loose, you are wasting your time and money.......I spend at least 20 hours fitting and assembling a short block, and that is just for a "stocker"-----a volvo turbo motor is NOT a "stocker"......it's gotta be tight, and it's gotta be right.....anything less is crap with a capital "S".........taking on a B23FT for your first rebuild is going to be a challenge; there aren't very many people I would trust to do a stock motor like a small block chevy, much less a high performance motor like a volvo turbo motor......that's why I build my own......good luck, cuz you're gonna need lots of it

First off, pistons are not round when cold - they go circular when warm. (if you look underneath the crown you'll see steel expansion strips cast into the aluminium or there'll be expansion control slots in the skirt, usually at the bottom of the oil control ring groove) They are generally "barrelled" also, (i.e. wider toward the middle, the top and bottom being narrower) so it is important where clearance in the bore is measured on the piston. The only really accurate way is to measure the bore precisely using an internal gauge then transfer that measurement to an accurate micrometer. Then measure the piston about two thirds down the skirt at right angles to the gudgeon pin. Take the first measurement from the second and divide by 2 - that's the clearance. Absolute accuracy is imperative and difficult to achieve without practise.

The actual clearance varies according to engine type but should be printed either on the boxes the piston kits came in or on the information sheet with them (if they came with them). Typically, its either 0.0002" or 0.0004", the greater clearance for injected engines which generally run leaner and have higher compression ratios than carb engines, consequently run hotter.

When you fit them, use a proper ring expander to fit the rings to the pistons, oil the piston and bore well, use a proper ring compresser, make sure the ring gaps are staggered (including the rails of the oil ring set), that the top ring gap faces away from the exhaust valve, no gap is on the thrust side, and the ends of the oil ring set expander aren't overlapped. Put some electrical tape over the big end bolts also to avoid the possibility of marking the crank with them when pushing the piston and rod into the bore. When fitting the piston, it takes a bit of a push but don't force it - if it doesn't want to go pull it out and fe-fit the ring compresser. You'll hear the rings enter the bore with a light "click". Tap the pistons in with a wooden handle once the the rings are in the bores. Phew! hope that helps. Cheers.

How long is a piece of string? Seriously, without rings the piston should slip into the bore easily, when it is at the top of the bore you should't be able to significantly rattle it about in the bore. Without measuring, which your machine shop should have done, it is impossible to really describe it. However one crude way of telling, and you should do this anyway, is to measure the ring gap. Put the ring in the bore and push it into the lower part of the bore with an upside down piston. Then measure whether the gap between the ends of the ring meets the spec with a feeler gauge. In a good quality piston like the Mahle if the gap is too big, likelyhood is the bore is too big. It is two in the morning here and I can't remember the spec, it will be with the piston bumff, and in the workshop manual.

When fitting the rings be careful if you haven't done it before, they are very easy to break. Remember stagger the ring gaps round the piston, and keep the gaps away from the thrust side of the bore.

Regards,

Rene:

>B23 block back from the machine shop who bored it to the first oversize with Mahle B23ET pistons from Mike Aero (Unitek). How tight/loose are the pistons (without rings) supposed to be in the cylinders?

* The normal over-bore size is found outside on the pistonbox.

* The resuling piston-to-bore clearance in to be found on the piston top.

* The first oem B23 oversize is 96.31 and on a upgraded engine we normaly never go beyond 96.32mm.

NB: All clearances are to be measureed @ roomtemperature i.e. 20 degrees C.

Each individual pisondesign are to be meausured often differently.

The piston in question you have bought, i.e. the forged Mahle 03759/B23ET is, by the oem/green manual, to be measured 12mm from bottom and 90 degrees fron the piston pin.

(I strongly suggest that any competent engine shop machinist should be aware of these details. Or at least know that the details is what it is all about, i.e. getting the proper measurements BEFORE attempting to perform any machining at all...)

>Mine are not tight. They are not overly loose either though. I really don't know how it is supposed to feel.

I suggest it is as bad to have to narrow bore as well as equally bad to have a larger than necessary bore.

Come back with your findings!

Best regards

Mike

Luleå, north of Sweden

OK, I went and measured it and I can fit a 0.05mm (0.001969 in.) feeler guage in the cylinder with the piston. The book says that the running clearance for the B23 is 0.05-0.07mm (0.0020-0.0028 in.) Is that what I'm measuring? What's running clearance? BTW: The pistons are market with 0.06, what does this mean? It's suppsed to be the first ovesize. I don't want to do a lousy rebuild.

Rene