Turbo M20 2.8L Stroker w/ 135mm rods

[SLEEPYDUB]

See Post 55

[digger]

Gapless works in boosted applications, but not sure anyone has experimented with gas porting the domed b25 design.

Gapless top does, 2nd isn't so good as the top ring seals worse . You won't find any data on gapless vs conventional other than hearsay as people keep some stuff to themself. Often you will hear people compare conventional and gapless but going from thicker conventional to thinner gapless which tells you nothing about the gapless part. Sometimes you will see data on end gap vs hp vs blowby and it's seems to suggest gapless shouldn't be worth much premium from a hp point of view. Gapless is good for reducing oil contamination so e85 and methanol it's helpful

[dvallis]

LoL. Digger and ForcedFirebird probably have 10,000 hours engine building experience between them. I would listen to their advice.

My spreadsheet above is a decent approximation of all the stroker data I could find around here. If either of you guys want to tweak the numbers, send me an update. I'll host it and repost.

[ForcedFirebird]

Gapless works in boosted applications, but not sure anyone has experimented with gas porting the domed b25 design.

[digger]

Yeah they make more money on gapless, ask them for conventional they have a nice 1mm 1mm 2.5mm with 2nd being Napier. You can just look in their catalog find a size with several options decipher the difference and ask them which is best while telling them the purpose

[ForcedFirebird]

Yes, but if you call them, they will try and sell you gapless for nearly every application. Stay with a conventional top ring and napier second (which is OEM on BMW's anyways).

[digger]

Also best to look at what rings are available, some sizes have more choices. Total seal are great

[ForcedFirebird]

The pins on the m20 are heafty and can be smaller diameter, BMW did it on the s54. Just broke two of them for turbo rebuilds, pretty excited to flow the heads personally, but that's for another thread.

[ForcedFirebird]

Without question bore it at least .25mm, but .5 is more common, +1 gives you matching gasket size to bore, which is great to avoid edges and hot spots. I have had blocks measure out for reuse of stock replacement pistons after deglaze, but no quality machinist will hone a used block the extra few thousandths for forged replacements. The bores will not be round. Don't forget to rent a torque plate, your local v8 shop probably won't have an m20 setup. These long siamese engines will move when the head is bolted.

[digger]

I am using ARP Main Studs. When you say it will need to be align honed, what exactly needs honed? The bore of the mains where the crank sets in? Sorry for the noob questions =(

As for needing the pistons before he bores the cylinder, I was under the assumption that I need him to tell me what size bore I need first? Basically to determine if the engine even needs to be bored out? I am trying to stick with stock size bore, so what is the proper sequence of events here?

He needs to tell you how much he needs to remove to get the bores good. I'd just order 85mm that's enough meat to true it up to keep it simple

[digger]

Yeah.
Could you quote an offer from MM for a block decked 0.5 mm?
No.
It is hard enough (and expensive) to get pistons that will work with 89.6 mm crank and 135 rods.
Get into the real world.
EngineBuildingBird will tell you that through all these years of engine building of his, he will deck the block and the head to achieve CR or quench.
I admire his reputation over this forum but sometimes you have to go with commonsense.

It's common sense of you take say 0.5mm from the block then you are going to be stretched slighty further to get the ring pack within the compression height avoiding the pin and valve reliefs. Most use 135mm rods so its not a big deal in the real world and it's not a show stopper at that point. A thicker gasket is fine to compensate, though not ideal on turbo engine especially is max effort is required. You can actually design a 3.1L piston with 135mm rod with oil ring not into the oil ring if you want.

[apostate]

Sticking with the stock size bore is a lottery.
Wait... EngineBuildingBird will say the final word, of course. ;)

[SLEEPYDUB]

Are you planning on upgrading the main cap fasteners? If so, they will need to be align honed - people think they don't need to, but the bore does measurably distort. Main bearings set to .002" and rods .0015, .005" thrust.

Make sure you have the pistons before he bores, as each MFGR will have a clearance standard based on material.

I am using ARP Main Studs. When you say it will need to be align honed, what exactly needs honed? The bore of the mains where the crank sets in? Sorry for the noob questions =(

As for needing the pistons before he bores the cylinder, I was under the assumption that I need him to tell me what size bore I need first? Basically to determine if the engine even needs to be bored out? I am trying to stick with stock size bore, so what is the proper sequence of events here?

[apostate]

Yeah.
Could you quote an offer from MM for a block decked 0.5 mm?
No.
It is hard enough (and expensive) to get pistons that will work with 89.6 mm crank and 135 rods.
Get into the real world.
EngineBuildingBird will tell you that through all these years of engine building of his, he will deck the block and the head to achieve CR or quench.
I admire his reputation over this forum but sometimes you have to go with commonsense.

[ForcedFirebird]

He's using solder to check the piston to head Squish band clearance during mock up.

You can still get a custom set of 3.1l Pistons 135mm Rod with a block decked 0.5mm from previous adventures. Note that MM use a 138mm rod with 89.6mm stroke work out the compression height on that

Not to mention BMW built machining into the design for future rebuilds. I don't do the full .5mm on the budget 2.8's, ~2.5mm (.010" to my tooling) which leaves the block still within factory rebuild limits.

If you purchase a reman from BMW, I can't see them NOT decking both the block and head, it's standard rebuild procedure.