I'm planning to overbore the engine cylinder by 0.5mm and install a +0.3mm head gasket.

This may have been looked at by someone here, but I couldn't seem to find it. I've taken the B46 motor out of the '15 cooper S during the weekend. Did a complete removal, now that the crank is out, I'm exploring options to putting it back together with beefier components in anticipation to the modifications I'm planning on doing.

Background of my story (you can ignore this paragraph if you don't care why I come down to tearing down the engine)
The motor had gone through a flood damage before so crank wouldn't turn. Therefore, I've automatically assumed it to be a bent rod or something along the lines of locking up the crank. Well, as I tear everything down to the oil sump pump, I realized the moment I removed that pump, the crank is now able to turn. A further look into the pump I find the pump has sucked up some water (in our case, it's that oil/water mixture much like milkshake). So it's actually the oil sump which is hydro-locked. The pistons/rods/crank all seem to be fine as I pull them out, I did some inspecting and nothing major is wrong with them. But I did have to tap on the piston to wiggle it free and tap on the rod fork to free it from the crank. Long story short, piston face has slight pitting, and rod end I'm not sure if it's still serviceable for re-installation. The most major problem I come across is I found some rust on the cylinder wall, they aren't deep, but I couldn't de-rust them by using a dish washing scrub nor by using copper wire wheel on a drill. So this brings me to deciding to cut the cylinder wall and get past the rust. AND it just so happens that I can go with a +0.5mm overbore seeing there are 82.5mm pistons offered by SuperTech.

Back to the plan:
I find SuperTech offering the 82.5mm piston versus the stock 82.0mm piston, so this will match the fit for an overbored cylinder. Searching for head gaskets through ECSTuning, I also come across a +0.3mm head gasket. Combining both modified dimension shouldn't be an issue as these aren't crazy size increases; but working the mathematical part of it below is what I get:

Parameters:
Before:
Bore=82mm, Stroke=94.6mm, Volume=499.584cc (if you multiply this by 4, you'll get 1998cc which is the displacement for our B48 2.0L engine)
I've also used the compression ratio of 11:1 and back calculated the spherical volume (dome) from the B48 head, it's 50cc, This yields the Compression ratio calculation of (500cc+50cc)/(50cc), which comes out to 11:1 as our compression ratio. This also gives me an idea that the height used in spherical calculation is roughly 21.7mm (this is important because it'll correlate to how added deck height will lower the compression ratio)

After:
Bore=82.5mm, Stroke=94.6mm, Volume=505.695cc (if you multiple this by 4, you'll get 2023cc)
Spherical Dome is now different due to the added 0.3mm deck height from the head gasket, with an equivalent 22.0mm deck height, new dome volume is 51.19cc, plug in the parameters I get 10.88 compression ratio.
As far as volume increase, I now have 25cc from overboring and 1.19cc from dome volume, that's a gain of 26.2cc with a 10.88 compression ratio.


Below options and conclusion drawn from the calculation:
If I only overbore the cylinder by 0.5mm without doing anything else, I get 2023cc and a 11.12 compression ratio. (While I do like the extra volume, not a big fan of higher compression ratio knowing I'm looking to up the boost)
If I simply increased deck height by 0.3mm without overboring the cylinder, I now get 1999.19cc and 10.76 compression ratio. (This is possibly the best of all worlds in terms of adding 1.9cc volume and achieving a 10.76 compression ratio)
If I do both, overbore the cylinder by 0.5mm and adding the 0.3mm deck height, then I get 2024.19cc and a 10.89 compression ratio. (This is what I'm looking to do because of issues with rusted cylinder wall, but 10.89 compression ratio is still better than 11.0 going for higher boost).

Another interesting conclusion I draw from the above work is how does the JCW motor achieve 10.0 to 1 compression ratio on the same exact platform. If stroke and bore are exactly the same, then the only way they can achieve this is either having a larger head dome. Makes me wonder if someone could have a JCW head and a S head side by side, I bet we'll find that difference.


What I'm planning on putting together: (also trying to cut the budget down)
- 82.5mm forged aluminum pistons (found a set from a Taiwan maker for cheap)
- Carrilo Forged Steel Rods with ARP rod bolts (may go with the cheaper options of maxspeedingrods on ebay)
- Crank Bearing Cap bolts and Head bolts to be ARP bolts
- Forge FMIC (may go ebay)
- Forge Hard Boost Pipes
- MarioKart's Ceramic Coated catless DP
- AWE Intake system
- (I don't plan on upgrading the exhaust because I believe adding a catless DP will already increase noise, I cannot stand it being any louder than stock already, so absolutely no tolerance for aftermarket exhaust)

- And finally Tuning (stage 2):
My target isn't to go all out and max it beyond 300 whp, I'm doing the internals due to a coincidence that there are issues with the cylinder bore and I want to tune the car. And I did come across a few threads that on Bytetronik's stage 2 tune there are at least two cases of piston failure, I'm hoping to address it by using better pistons and consequently, if the final compression ratio is slightly lower, would it be a little safe for the tune. I haven't decided which tune to use, but most likely I'll be shooting for RPM tunes by MarioKart, the guy's been nothing but great and easy to work with. However, Bytetronik is in nearby town, and if I've already beefed up the engine, stage 2 with Bytetronik wouldn't be a bad go. After all, I'm just wanting to run a stoutly built motor with a slightly aggressive stage 2 tune as from an R56 MCS, the RPM stage 2 plus the JCW turbo had already done me plenty of fun. I'm looking for a slight more on the F56, not looking for the ultimate beast.

I know I wrote a long rap sheet here, but really hoping to get some pointers out here by some guru's. So thanks in advance for any help I can get.

 

Don't have time right now to try to work out compression ratio. But I'm thinking you can work from the stock engine bore/stroke to arrive at a per cylinder displacement and from the increase in bore size come up with a new cylinder displacement and from this come pretty close to being able to work out the new compression ratio.

If you want the compression ratio upped to closer to what it was you can remove some material from the block deck or the head.

However, while I have no MINI engine experience years ago I rebuilt an SOHC engine and the caution then and now is to avoid removing any more material from the block deck than is necessary to ensure the block desk is flat and straight. Likewise the head if it needs resurfacing to ensure a good gasket seal the minimum amount of material should be removed.

I do not recall if the head or block needed any material removed. I did go with oversized pistons but installed higher compression pistons. I don't recall now the change in the compression ratio from the overbore. The overbore was not much, maybe 0.015" (a bit less than 0.5mm) but I might have had to go to 0.030". It just depended upon what was available in oversized pistons back then. Also, the advice I received from my auto tech buddies was only go as big as necessary to clean up the bores to keep the cylinder wall thickness as thick as possible.

I used a new stock head gasket.

With a SOHC or DOHC engine any change in the distance between the cam centers have from the crankshaft center affect the cam chain slack. The hydraulic cam chain tensioner has some ability to take up the slack but it extends out more and this can result in accelerated wear of the tensioner piston.

 

Thanks for the great insight. I actually do want to overbore 0.5mm and increase the gasket thickness by 0.3mm to decrease the compression ratio on purpose. I don't intend to cut the deck and bring the compression ratio back up to 11.0. I rather let it be lower to relieve the tuned stress of having too high a boost. So my overall approach is en route to the intent. I suspect that if the block remains the same and by lowering the compression ratio and increase volume, we can allow a better safety boost increase factor; seeing from some of the failed pistons on stage 2 tuned cars. I also gathered that this is possibly why the JCW engines are designed with a 10.0 compression ratio and allowing higher boost to get to 230bhp from the factory. So I won't be looking to cut the deck, and therefore won't have any slacking issues with the timing chain, better yet, with a +0.3mm head gasket, I'd imagine it would tighten up the timing chain a tad more. But those are good points you're pointing out, I appreciate it. I think the main issue I'm trying to address is that I do want to bring this car to an aggressive enough stage 2 tune and not blow the piston away like some fellow members have experienced.

 

My memory of the math of increased bore size is you won't reduce the compression ratio that much by increasing the bore size. The combustion chamber size doesn't increase. And changing the head gasket thickness is not advisable.

In case of an N/A engine being turbo/super charged if the boost level is targeted much above 6psi low compression pistons need to be fitted. To give you an idea of how much the compression ratio needs to come down: One of my Porsche cars, with a N/A engine, had an 11.2:1 compression ratio. The other one came with a turbo charged engine (with two turbos) and the engine had around a 9:1 compression ratio. But the turbo engine was boosted to nominally 0.7 bar (10.1psi) but under some conditions -- like operating at higher elevations -- boost could reach 0.8 bar (11.6psi) or even 0.9 bar (13psi).

If you plan on running boost levels above say 6psi you need to seriously consider low compression pistons. Maybe running proper low compression pistons you can avoid your engine suffering the same fate as your fellow members who apparently tried to run too high a compression piston with too much boost.

 

That's exactly on par with my planning. However, for the B48 engines, I don't find a low compression piston, yet. Maybe the Rods and the Pistons as a set from a JCW B48 can be the candidates, but I'll have to dig further into the specs. One very good thing about these B-series engines is how modular they are. I've been searching up parts in the B58 platform and turns out there are more motor-works being modified on those than the B48. At this moment, this motor doesn't have that much aftermarket mod support besides the simple bolt-ons. Therefore, as in my case where I have no choice but to overbore the cylinder by +0.5mm anyways, I'm limited to what I can find. But I'm on the same alignment as you, I'm looking to rebuild the engine with forged internals, and if it so does lower the compression ratio, that's a plus. If not, at least with stronger internals, it won't fail that easily with more aggressive stage 2+ tunes. Thanks anyways.

 

With an overbore and the same size combustion chamber I believe you are increasing the compression ratio a bit. The cylinder is larger, has more volume, but you are still cramming this into the same size combustion chamber.

If the compression ratio and boost are too aggressive no piston/rod will survive. The engine will detonate itself to pieces.

 

Good thing you're putting it this way now. It's what I'm trying to get some insight on. I still couldn't figure out if I'm overboring by 0.5mm and adding head gasket height by 0.3mm if that's going to increase or decrease the combustion ratio. Will take this to the local engine specialist for verification this week. Thanks RockC, I'm trying to make sure what I"m doing is to decrease the compression, not increasing it, the same alignment to what you're mentioning for piston/rod safety on higher boost.

 

Well folks, I haven't been able to find much DIY info either through the forum or on youtube, so I think this will be a good point to gather my work and start documenting it here as a collective spot for public share and use. I'm attaching some pictures of my work along the way:


As stated before, the car's gone through some flooding, engine sucked in some water and seized. I dug right hoping without having to take the engine out of the car I can try to un-seize the motor. Well, I ain't that lucky. No matter what starter fluid or penetrating fluid I put in the combustion chamber I couldn't hand-crank it to move nor could I get the starter to crank it under full battery charge.

(Assessing Flood Damage)

I decided that the motor must come off and I'll have to take the head off and check out the pistons. Under NewTis.Info, you're supposed to take the engine out from under the car. Well, I found a way to remove the entire front without opening up the HVAC system. It's quite simple, bumper and intercooler out first, then just dig right in to remove the entire front end top and bottom, include the front end support all the way to the subframe connections. Doing it this way I learned that I can leave the output shafts on the transmission and with an engine hoist, I can just slide the motor out from the front end. I'm very sorry that I didn't take more pictures along the way it's because I got real smooth with the removal and just went all the way with it. Besides my gloves are dirty and I didn't want to touch my phone to take pictures that way.

(Engine Out pic 1)


(Engine Out Pic 2)

It's getting dark, so I slid the engine into the garage:

(Engine in Garage)

Close-up view of the engine with head removed. There are some carbonation with it, still couldn't turn the crank at this point. I find the exposed part of the cylinder wall having rust. I was suspecting excessive rust would cause the assembly to seize, or worse, I was afraid I would have a bent rod or something of that sort:

(Engine in Garage - Close-up)

Calling it a night, back in a week to continue:

(Parts laid out in garage)

Back at it again, transmission pulled off, oil pan off, crank flex plate removed, oil pump removed, piston removed, (able to turn the crank after the oil pump removed, and all rods are still straight). Crank turns freely now that all pistons/rods are out of the block: (cylinder wall still has rust, I use copper wire wheel to brush it with WD-40, removed some rust, but there are rust-roots deep into the cylinder wall; looks like I'll have to send this off to a shop and get them to overbore it by 0.5mm)

(Short Block Mounted on engine stand)

Cleaning up the interior and exterior of the short block in preparation of sending it to the shop for overbore service. At this point, I've gotten on the drawing board and planned ahead accounting for all the parts I'll need to buy to put this motor back together. It'll be about $2,700-$3,000 to piece it back together with upgraded 82.5mm forged pistons and forged rods + ARP bolts + stock gasketing and misc parts. Going hunting for an engine in the junk yard.

(Assessing the Cylinders - Rusted)

Junk yard engine arrived: My flooded car has appx 63,000 miles. This junkyard engine has 34,000 miles. They cracked the oil filter housing when getting this thing secured onto the pallet...Frustrashe...

(Replacement Engine hoist off the van)

Prepping the donor engine: (Walnut blast the intake valves, the exhaust valves, and the cylinders, to do the cylinders, you'll need three vacuum cleaners to all provide the suction as you spray walnut into the cylinder)

(Prepping the Replacement Engine)


I'll be mounting the transmission next week and will be installation the catless DP from MarioKart before bringing it back into the engine bay. Other upgrades: Forge FMIC and Hard Boost Pipes. I plan to only tune it to stage 1 on this car, so will be keeping the engine internals stock. The rusted engine I'll use it for a build to have the forged internals and plug it into the next F56 I plan to get by mid-year 2020.

I'll add more pictures and updates to my progress.

 

It's confirmed, with an overbore and increased head gasket height, the compression ratio will be lower. I've reflected this conclusion on the original post. Basically, overboring the cylinder by itself will increase the compression ratio, but adding that 0.3mm deck height will offset it quite greatly and bring it down to about 10.88.

I have just also acquired direct communication to dealers in Taiwan for B48 special tools, engine components, rods, and pistons. Can get them at a cheaper price than ECSTuning and eBay. Awesome.

 

It turns out I won't be needing to overbore the cylinders, simple honing tool put all the rust away for good. I'll be sticking with the regular size 82.0mm forged pistons. And with 0.3mm added deck height from the head gasket, target compression ratio will be 10.76 which will be ideal for going up the boost.

 

Any updates yet? I'm also planning to get my engine forged but i'm not sure what all has to be done. I'm going to use my current engine (85k miles) stage 2 (300hp) no problem at all.
But after a year i'm looking for more power. Torque is going to be the limited factor due to the automatic transmission. Hoping for 350+hp

 

I've decided to just run my current project with stock motor components along with upgraded catless DP and a intercooler with stage 2 ByteTronik tune. The extra motor I'll slowly build the forged internals and good news is that I won't need to overbore the cylinders, so i can stick with the original 82mm forged pistons. Since I'll go slow on this project, I've lots of time to research and decide. Supertech, Wiseco, Carrillo, JE, Mahle, and Ross are all players making the forged pistons. If you look into B58 pistons, you can actually make it work, too. Just be aware of your final compression ratio with what you're buying, and do your calculations. For our B46/B48 motors, I'd suggest you target your compression ratio between 10.0 to 10.5 to leave some room for higher power and still have a good safety net. The 11.0 ratio on the non-JCW platform in my opinion is a tad on the higher end and leaves little room for upping the boost especially with the factory pistons. By all means, the factory pistons aren't bad if you're targeting below 250whp. But if you must go above 250 on the 11.0 compression, just be sure to develop a good driving habit. Punching it down between every stop sign will likely get you some undesired outcomes.

 

i thought Supertech makes pistons that are 10.2:1 compression. On Lohen they even offer fully build B48 engine's. GP GARAGE in Italy offers different kind of pistons. You can check them out. I'm @ 265WHP now. Stock pistons. In my experience it's fine. It's more than just the map. You need specific oil that helps eliminate pre detonation, excellent fuel (here we have high quality RON98 e5.) Those B48 mainly has a problem with broken rings that destroy the piston itself due to pre detonation/high combustion temperature's

 

Supertech over here in the US, according to what I've found so far makes two version, the 11.0 and the 10.5, not sure if they have the 10.0 CR available to us. But yes you're right, it's more than just the map, it's the overall condition of the car, driving habit plays a big role, well-kept maintenance, high octane fuel, and most importantly, anything you can do to eliminate pre-det. In regards to B48 mainly having problems with broken rings, it is additionally due to the ring deck spacing between the rings. The B-series motors are variant descendants of the N-series motors. In some way, better and stronger (close-deck housing and more boxed geometric rod/piston/crank assembly being the main change), but in others, it did take some sacrifice to achieve cost cutting in modular linearity manufacturing.