Boring pistons worth it?

[tooshfty]

Im considering rebuilding my engine and replacing stock stuff with forged internals and fresh stuff. But when I looked at pistons I noticed there are a bunch of bore options apart from stock. After I add forged internals Im thinking about getting boost and going for power. Does Bore add any advantage for safety or anything like that? My idea is essentially to run forged internals with some cams until Im ready for boost.

[gdavid]

Generally, you don't want to bore out the s52 engine unless you have cylinder wall damage, then you would go up in size as needed to match the bore of the block. There just isn't much material between cylinders with the s52 to begin with and you are limiting your ability to rebuild down the road. Some engines have more space between the cylinders and therefore more room to grow but it wouldn't make sense in intentionally overbore a healthy s52 you intend to turbo, you will be asking more of your head gasket. If you are going with custom pistons and building it up immediately with a turbo, it would make sense to go with a shorter piston/rod combination to lower the compression ratio (which will be more than made up for with boosted intake pressure) if you aren't adding the turbo immediately, going together with stock dimensions and lowering your compression ratio later with a thicker head gasket makes more sense. In fewer words, changing the piston diameter does nothing to make it "safer", the compression ratio would remain the same.

[tooshfty]

Generally, you don't want to bore out the s52 engine unless you have cylinder wall damage, then you would go up in size as needed to match the bore of the block. There just isn't much material between cylinders with the s52 to begin with and you are limiting your ability to rebuild down the road. Some engines have more space between the cylinders and therefore more room to grow but it wouldn't make sense in intentionally overbore a healthy s52 you intend to turbo, you will be asking more of your head gasket. If you are going with custom pistons and building it up immediately with a turbo, it would make sense to go with a shorter piston/rod combination to lower the compression ratio (which will be more than made up for with boosted intake pressure) if you aren't adding the turbo immediately, going together with stock dimensions and lowering your compression ratio later with a thicker head gasket makes more sense. In fewer words, changing the piston diameter does nothing to make it "safer", the compression ratio would remain the same.

Thanks for that bit of info. So if I'm rebuilding for turbo later it would be safe to do a thicker head gasket and forged internals with stock dimensions? Getting a thicker head gasket would lower the compression ratio correct?

[pbonsalb]

Stock S52 is 86.4mm. Bore to 86.5 or 86.6 and use JE or Wiseco or CP or Mahle. Specify 9.0:1 for turbo on pump gas. Use Eagle or K1 rods since stock ones are not reliable past about 500lbs rwtq. Use a JE cutring head gasket -- no spacer since you dropped CR with the pistons.

[tjm3]

For a few dollars more JE will make the forged pistons to whatever diameter you want so if you go 2, 3 or 4 thou over you can hone a nice finish and straighten out any taper or out of roundness in the cylinder. The JE forged need a lot of clearance being low or no silicon pistons, and they'll recommend more for turbo application. They'll swear they won't rattle when cold, but.....

[pbonsalb]

JE has off the shelf in 86.5mm and 9.0:1. I had an M52 block bored for the 86.5 JE 9.0:1 pistons a few years ago. You could use an S52 if in good condition, but that would be starting with 86.4 bore so only a 0.1mm overbore and if 1mm is 0.040 inches then it is only 0.004 inches overbore. Your bores would have to check out fine to go so little. Final specs depend on the piston, the bore and the clearance, obviously. Even with the BMW factory S52 pistons, I think there are several slight variations of bore.

[tooshfty]

JE has off the shelf in 86.5mm and 9.0:1. I had an M52 block bored for the 86.5 JE 9.0:1 pistons a few years ago. You could use an S52 if in good condition, but that would be starting with 86.4 bore so only a 0.1mm overbore and if 1mm is 0.040 inches then it is only 0.004 inches overbore. Your bores would have to check out fine to go so little. Final specs depend on the piston, the bore and the clearance, obviously. Even with the BMW factory S52 pistons, I think there are several slight variations of bore.

So it would be safer to buy a slightly higher bored out piston set?

[tooshfty]

To be safe what would you guys recommend for bore size so we don't run into the problem of undersized pistons? Also someone mentioned thicker head gasket with more bored pistons does that work? It would lower the CR but by how much?

[pbonsalb]

I think on an S52 you want the minimum overbore to provide the best sealing surface for the gasket. There is very little between the cylinders. Some say it does not matter and just go for the 87mm bore pistons that require enough overbore to take care of most issues you might find. But if you took the motor apart and inspected it, a shop might say 86.5mm would work. You would not know without having a machine shop inspect the bare block. Up to you. I sourced an M52 block and put my S52 crank in it and K1 rods and had it bored to fit JE 86.5 9.0:1 pistons. If you want to be safe on using your block without inspecting it, the 87mm pistons are the way to go, but you lose 1mm between cylinders.

You can lower compression either with pistons with a bigger dish or with a thicker headgasket, but you do not do both. JE and the other piston makers sell pistons with 9.0:1 CR. They actually do not sell stock CR pistons, but rather 11.0:1 pistons although you can pay extra to special order whatever you want. If buying pistons, why buy a thicker HG or spacer? Just get the lower CR pistons. A CES Motorsports cutring gasket and copper spacer will drop CR from 10.5:1 to 9.0:1. Same with a Cometic 0.140 MLS.

I ran the cometic 0.140 MLS on an S52 for several years and it did fine, but that was years ago. 99M3 and this was in 2010-2014 and the engine was low miles. Today, an S52 is about 20 years old and may be very high mileage and probably not as likely to hold up long term with 2x or 3x the power and just a thicker HG. Again, your choice. If you are a DIYer, its just your time and some money. If paying a shop, do it right so you don't have to pay labor again. Today, I would use a cutring gasket over the MLS. I have a cutring in my fully built turbo S52 99M3.

Its not cheap to fully build a motor. Can be even more expensive to fully build and port a head. I would consider your budget before you dump a bunch of money into an old E36 that you will never get back out of it. Another great option is to sell the E36M3 and buy a car that is newer and faster to begin with. I really like the 135i and 335i with N54 engine -- you can find a 335i for $10k to $15k. I have an 08 535i with the N54 and just a JB4 tune and it goes pretty good. You could also look at the E46 M3 in the $15-25k range. If you stretch a little more to $25k plus, you can look at E90M3 -- I have an E90M3 with bolt on mods and like it although power is very different from a turbo car in that you have to wind it out to get the fun power.

[tooshfty]

I think on an S52 you want the minimum overbore to provide the best sealing surface for the gasket. There is very little between the cylinders. Some say it does not matter and just go for the 87mm bore pistons that require enough overbore to take care of most issues you might find. But if you took the motor apart and inspected it, a shop might say 86.5mm would work. You would not know without having a machine shop inspect the bare block. Up to you. I sourced an M52 block and put my S52 crank in it and K1 rods and had it bored to fit JE 86.5 9.0:1 pistons. If you want to be safe on using your block without inspecting it, the 87mm pistons are the way to go, but you lose 1mm between cylinders.

You can lower compression either with pistons with a bigger dish or with a thicker headgasket, but you do not do both. JE and the other piston makers sell pistons with 9.0:1 CR. They actually do not sell stock CR pistons, but rather 11.0:1 pistons although you can pay extra to special order whatever you want. If buying pistons, why buy a thicker HG or spacer? Just get the lower CR pistons. A CES Motorsports cutring gasket and copper spacer will drop CR from 10.5:1 to 9.0:1. Same with a Cometic 0.140 MLS.

I ran the cometic 0.140 MLS on an S52 for several years and it did fine, but that was years ago. 99M3 and this was in 2010-2014 and the engine was low miles. Today, an S52 is about 20 years old and may be very high mileage and probably not as likely to hold up long term with 2x or 3x the power and just a thicker HG. Again, your choice. If you are a DIYer, its just your time and some money. If paying a shop, do it right so you don't have to pay labor again. Today, I would use a cutring gasket over the MLS. I have a cutring in my fully built turbo S52 99M3.

Its not cheap to fully build a motor. Can be even more expensive to fully build and port a head. I would consider your budget before you dump a bunch of money into an old E36 that you will never get back out of it. Another great option is to sell the E36M3 and buy a car that is newer and faster to begin with. I really like the 135i and 335i with N54 engine -- you can find a 335i for $10k to $15k. I have an 08 535i with the N54 and just a JB4 tune and it goes pretty good. You could also look at the E46 M3 in the $15-25k range. If you stretch a little more to $25k plus, you can look at E90M3 -- I have an E90M3 with bolt on mods and like it although power is very different from a turbo car in that you have to wind it out to get the fun power.

My car is 190k miles, and I do agree that the best course of action is to just buy a faster car. However, I have no intentions of selling this car which is why i want to invest money into it. I really appreciate your input however. That is a lot of useful info. Would you say there are stronger head gaskets than stock that are similar dimensions?

[pbonsalb]

There are 2 standard replacement HG. People say Elring is better than Victor Reinz. Both are stock spec 0.71mm, I think. With a resurfaced head, CR will be very slightly raised due to the decked head. Cometic sells a 0.080 and JE sells a 0.079 cut ring. Both are thicker to compensate for decking the head. My view is the JE is the best sealing HG. But an Elring is much cheaper and fine for anything but a high boost motor. I would recommend ARP head studs. They cost much more than stock bolts but hold boost better.

It's not investing money. It's spending money. As long as you enjoy the results blowing money on modding cars can be worthwhile.

[tooshfty]

There are 2 standard replacement HG. People say Elring is better than Victor Reinz. Both are stock spec 0.71mm, I think. With a resurfaced head, CR will be very slightly raised due to the decked head. Cometic sells a 0.080 and JE sells a 0.079 cut ring. Both are thicker to compensate for decking the head. My view is the JE is the best sealing HG. But an Elring is much cheaper and fine for anything but a high boost motor. I would recommend ARP head studs. They cost much more than stock bolts but hold boost better.

It's not investing money. It's spending money. As long as you enjoy the results blowing money on modding cars can be worthwhile.

Thank you again. And to me spending the money is investing it because the outcome will be better than the value of the money itself, which is why i want this car to be insane hahaha

[pbonsalb]

I have been modding mine for 12 years and 90k miles. There have been some ups and downs but I guess I would not still have it if it was not worthwhile.

[tooshfty]

I have been modding mine for 12 years and 90k miles. There have been some ups and downs but I guess I would not still have it if it was not worthwhile.

Aside from parts, what would labor look like if i were to swap the internals: down to pistons connecting rods, bigger cams, etc. Maybe a ballpark number, just something to work with

[pbonsalb]

There are too many variations to the work. Are you paying a local mechanic to remove and install the motor or just a machine shop to bore and assemble the shortblock? Or to assemble the complete long block? If the latter, what work are you having done to the head and what valve train parts (guides, valves, springs, retainers, lifters) are you replacing?

An autohead or vac refurbished head with supposedly checked valves and springs and retainers might be $800 and does not come with lifters, cams, cam trays or vanos.

You can price top and bottom end gasket kits at ECS or TCL.

Pistons and rings and rods might be $1400. Try eBay to see if there are any packages. Get part numbers and Google for prices.

You need bearings, chains, guides, rebuilt oil pump or rebuild kit, rebuilt vanos or rebuild kit. Little stuff can add up.

Machine shop disassembly and cleaning and testing and balancing and reassembly of the shortblock could be $1000 to $1500. If it assembles the complete long block and times the cams, probably $2000-$2500. The machine shop could refurbish your head, maybe for less than vac or autohead but it depends on what your head needs.

There are usually some surprises along the way. It's not typically ahead of schedule and under budget. And opportunities for upgrades. Fancy oil pump shaft? Upgraded fasteners? Better head gasket? Porting? 5 angle valve job? Stewart water pump? With you doing the mechanical work, you would still spend $4k to $8k or perhaps even more.

You also read about people doing the minimum -- just re-ringing the pistons, honing the bores, replacing beadings and gaskets, and just doing what the machine shop said was needed on the head. Those guys are DIYers who might refresh a motor for $2k.

If you are paying a mechanic to pull and install the motor and perhaps install the head, I'd add 16 hours of labor minimum and maybe 24.

[vollosso]

I always bore motors. 100% of the time.

If I'm building something I want a perfectly round and square bore. Old heat cycled motors have changed in shape and WILL NOT seal as good.

Your used block bored out will be a better block.

Fwiw I run 87mm on most things. With a cutring there is little to worry about.


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