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Reading Factory Specs

Practical Engine Building

Discussion and questions related to the course Practical Engine Building

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So i mic'd the piston(s) and set that same measurement to my bore gauge. reading the bore gauge, my piston to wall clearance is .002 and my out of roundness of the cylinder is at .0015. below i have posted the specs for my motor that im trying to go off of.

To me it seems that the factory is ok with .007874 of out of roundness (measurement difference from A to B on cylinder bore), which seems excessive. I feel that i am reading this wrong or not fully understanding. Please correct me. Out-of-round limit 0.20 mm (0.007874 inch)

engine is 4.6 2v out of 98 mustang gt, motor will be supercharged with saleen m90 blower and will make close to 400hp with all the supporting mods(cams, ported heads, headers etc.)

Cylinder Bore

Diameter f · Surface finish (RMS) 0.2-0.6 Microns

· Out-of-round limit 0.20 mm (0.007874 inch)

· Out-of-round service limit 0.70 mm (0.027559 inch)

· Taper service limit 0.70 mm (0.027559 inch)


· Diameter g 0.028 mm (0.0010 inch)

· Coded Red 1 90.185-90.205 mm (3.55-3.551 inch)

· Coded Blue 2 90.195-90.215 mm (3.55-3.551 inch)

· Coded Yellow 3 90.205-90.225 mm (3.551-3.552 inch)

Coated Piston-to-Bore Clearance -0.005-0.025 mm (-0.002/+0.001 inch)

Pin Bore Diameter h 21.008-22.014 mm (0.866-0.867 inch)

Almost 8 thou is an incredibly 'generous' out of round limit! Of course it's dependent on a particular manufacturer and the design of the block but my personal target is to see the cylinder out of round fall at or below 0.0005" (half a thou) although this isn't always realistic in every engine. To give you an idea of what other manufacturers deem acceptable, the factory limit for a Subaru FA20 is 0.020 mm or 0.00039".

What you need to take into account is that some blocks will distort when the heads are bolted down and in some cases this can be significant. It's possible that a known distortion is factored into that particular specification but I don't know enough about the engine to be able to judge. Ultimately with any out of round, less is more from a performance perspective and regardless of the factory limit I'd be questioning anything over about 0.001" but I'd also be checking this with a torque plate fitted. If you don't have a torque plate you could fit a cylinder head and use the bore gauge from the underside of the block as a sanity check.

that was an amazing answer and exactly the info i was looking for!! Thank you very much for taking the time to respond. i will double check my measurements, and also put a cylinder head on and recheck with the head on.

what do you recommend for ring gap for a forced inducted (supercharged) engine making about 400 hp with compression ratio at 8.5:1?

have you ever seen a cylinder with a positive reading? like +.0005, and if so what do you do about that? assuming have the block bored. example cyl size is 3.5520 and has a reading of 3.5525 or even 3.5530.

You need to check with the information delivered with your ring set to be 100% sure but I'd be expecting the top ring to be in the region of 16-18 thou. I'm not sure I follow with your 'positive reading'? Can you clarify a little more?

so i was getting a positive reading on the bore gauge, meaning the cylinder being smaller. i went back and bolted the main caps and cylinder heads on and re measured the bores and it checked out ok, so either i was doing some thing wrong or it was distortion in the block. my money is on i was doing some thing wrong.

say your cylinders are slightly out of round. maybe .0005-.001 out of specification, what kind of failure or issues will this cause in the engine. is this going to cause ring sealing issues or excessive wear/binding of the piston?

Generally a bore that is out of round will compromise ring seal slightly. You're also likely to have a small increase in blow by and potentially an increase in oil consumption. Obviously the magnitude of all of these things is dependent on exactly how far out of round you are. Generally I wouldn't worry about anything at or below 0.0005", and 0.001" Is probably still unlikely to really be an issue unless you're building an engine for a controlled class where every last hp is critical.

Those figures are certainly in the WTF range - did you guys note the 2 thou' interference fit for the pistons???

Just to clarify, this is a new/freshly bored engine or one that has some mileage on it? A used engine will have more wear on the major thrust side but also some on the minor with less on the line of the engine crank, as there is a side force reaction to the piston and rod angle.

Andre has covered my thoughts on possible designed distortion, but with that much I would expect there to be some major problems getting the pistons in, to say the least.

From your comment about a 'positive' reading, I would have suspected either a guage or operator error - are you SURE you have been doing it correctly, reading it correctly and that it is actually measuring correctly? BUT the apparent interference fit mentioned earlier could be a real thing - something I thought was restricted to some very specialised engines, such as in F1, where the engines are brought up to temperature with heating elements before being turned over.

Had a good sleep and, thinking on it, could these be the rough cast tolerances, before machining? Otherwise they just do not make any sense.. heck, even then they don't.