mahoney944

Well I've decided to rebuild my engine.... having it out of the car and torn apart, I have some scored cylinders. The block is going to need bored out and I'm looking at options for how little or how much I should bore it. My question is how much can you bore these engines before needing a new crank / rods...etc or just effecting timing and counter balancing in general? I see some good deals on Wossner 100.5mm and 101mm piston and ring sets but I can help thinking why not go bigger...structurally these blocks seem thin walled which is a concern. Any help / tips?

V2Rocket

The MFR of the material used in the blocks said no thinner than 1/4" cylinder walls.
101mm is no problem.
Wossner's are nice...don't worry about the piston weights, as long as they match eachother.

Paulyy

Bore it first then get the pistons to match size wise.

V2Rocket

You can't bore it right without having the pistons in hand to measure each one exactly...100.5 or 101 is still a nominal size, the bores should be tailored to suit each piston for best fit.

Paulyy

This is what i've been told a few times anyway.
you in this case the over bore is 0.5mm or 1mm.
So you'd bore it 0.5mm oversize. if something goes wrong for what ever reason, you go to 1mm. then you'd put the correct size pistons in.

If you've got the 0.5+ size pistons, then the bores are made to big for what ever reason. You need a new set.

V2Rocket

You take your bare block and measure the bore roundness/taper and figure how much overbore is needed to make the cylinders straight and round again.

Then you order pistons in that nearest nominal size, rounding up (100.5 or 101 in our case).
When the pistons arrive you measure the pistons to get their actual diameter, figure target bore clearance, and then bore the cylinder to match.

So a "100.5" piston may actually be 100.4 in diameter and you put say 0.08mm clearance into it on either side, so you bore the block to 100.56.

Tom M'Guinn

I took the Jerry Woods/Bruce Anderson 911 engine building class many years ago and expecting to see how he sized pistons/gaps, etc. If memory serves, the class's engine was a 2.2L 911T. When they got to that part, Jerry just took new cylinders and pistons out of PCNA boxes and bolted them on. He said he used to check clearances, etc. but after so many years of having them perfect right out of the box, he didn't really see the need for a stock street motor. For all other motors and builders, they all seem to do it the way you describe above -- get the pistons in hand, measure them up, and drill the block to fit.

mikey_audiogeek

Either approach can be "correct".

When you're dealing with a piston manufacturer who can't "make to size" then you get the pistons first and bore the block to match.

However if the piston manufacturer has very tight control over tolerances and can make to suit, then you can bore the block first and then order the pistons.

Fundamental approach: do the step you have LEAST control over, first!

mahoney944

Thank you for the replies. So at what point would the bore sizing effect the balancing of the crank and balance shafts or even timing? I'm trying to decide if I should just take as little as possible to make the bores straight and true again (say 100.5mm or 101mm) or make them as big as I can on the stock crank. I guess I'm trying to say how big of a bore can I get away with without having to change the rest of the internals

V2Rocket

The crank isn't balanced against the rod/piston weight on an inline-4 engine.
The rods/pistons balance each other out.
The Wossners have good quality control and should be very close in weight - you just put a piston on a rod and weigh the piston-rod combination and make all 4 as close as you like.

Don't bother messing with balance shafts or timing...not enough difference to matter.

rlm328

Send it to LR and have him bore it and match to pistons. The material that the block is made of takes special techniques that most machine shops do not have.

And yes if you change the piston ODs you can keep your present internals. It is when you change your stroke that things start getting interesting with internals. The mass added due to a small OD change is pretty insignificant.

mahoney944

Shipping from PA, there and back, makes it hard to justify for me. Plus I'm a turbo machinist myself lol.

Which leads into my next question. Windage ports... I see them between 1 and 2 then between 3 and 4. What about between 2 and 3? Looks like it in this pic unless it's in there factory.

V2Rocket

If you have a shipping or freight account it's super cheap. I brought a 951 short block from Cincinnati to Los Angeles for $162.

V2Rocket

A bare block can ship ups or FedEx ground

PorscheFanatic202

UPS charged me 120 and they picked it up from my house. I sent it in a home depot black and yellow crate