Internal Combination Options

[flyingbrickperformance]

I'm starting to look around at the various internals to build up an M42.  After some research, it appears there are several different routes, but many require machining pistons or custom pistons.  As with my prior M20 stroker, I'd like to go the route of a mildly stroked bone stock bottom end that requires nothing more than standard machine work, i.e., reconditioning and maybe boring.

After doing some research, I'm intrigued by the potential of mixing and matching the M20 and M40's components to acheive a decent mongrel build and wondered if anyone had spent time looking into this.

My calculations and specs show that the M40 rods at 135mm matched to the M44 crank at 84mm with original spec M20 eta pistons would increase stroke slightly, lower the rod ratio slightly, and put the piston crown 0.7mm over the deck height - which might only require some minor shaving.  Piston shapes seem to be comparable from what I've seen, and the bore and pin sizes appear the same from my specs.    

I have S50 pistons but using them requires maching them down to accept the 140mm rods, or having a set of 137mm rods.  

The only other option I can see is to use the same combo with the S50 pistons, but have the pin move up but this can only be done about 1mm probably, and decking the block another .3mm which should get to at least 0.0mm over deck, so it still doesn't help much and it will likely cause problems long term.

Anyone ever tried to use the M20 pistons to midly build up the M42.  I figure with some slight head shaving, pin relocation, ITB's etc, you could have a pretty nice little screamer that doesn't really cost a significant amount of dough when compared to some of the engine build-ups I've seen.  I don't have any experience with these so my background is in the M20 world, but I suspect the principles hold true for both to some respect.

[christophbmw]

whats the bore on the M20 pistons, they key on the M42 is to not go above 86mm bore. also what is the weight of the ETA pistons (sorry dont know the M20 specs off the top of my head), just curious. i am planning a build to. the M44 crank has almost 3mm greater stroke than the M42 (do you have an M42 or a M40?). the M44 crank, along with a slightly bigger bore and some mild camming will get you in the M20 stock power range.....maybe a bit more.

[flyingbrickperformance]

Not sure on weight, but bore is the same - 84mm.  I was looking to go 86mm using the S50 but I'm starting to wonder if my money would be better spent on the head, using the m44 crank and M20 pistons to get a longer stroke without much hassle, and do more with the head.  I'm not looking for killer power since its going into a 1200lb car.  I'd rather have a solid bottom end that'll go to 8500rpm to make use of an ITB setup and good cam specs.  Since I don't have much in the way of emissions requirements with the project, I can do a lot that might not work well with your normal e30/e36 application.

[christophbmw]

M44 crank and M20 86mm pistons give you lots of room to work with on the head. the M42 head flows VERY good to start with so some nice cams, and maybe a porting job could get you in the 170hp range easily with a very nice powerband. i would focus on the head rather than spend the money for the M47 crank.

[Boyracer]

Yes M42 head is quite good. I just read about tuning of Rover K series engines (largest capacity 1796 cm3) and their largest possible oversize intake valve is 33.7 mm. M42 stock intake valve is 33 mm... Actually M42 has larger exhaust valves than K series engine intake valves! :p

I think bulletproof and well balanced bottom end allowing stratospheric revs matched with well flowing head is a good goal to go for. But to use very high revs you need long duration cams which can be a problem in emissions sense.

[flyingbrickperformance]

Yes M42 head is quite good. I just read about tuning of Rover K series engines (largest capacity 1796 cm3) and their largest possible oversize intake valve is 33.7 mm. M42 stock intake valve is 33 mm... Actually M42 has larger exhaust valves than K series engine intake valves! :p

I think bulletproof and well balanced bottom end allowing stratospheric revs matched with well flowing head is a good goal to go for. But to use very high revs you need long duration cams which can be a problem in emissions sense.

Like I said, emissions aren't an issue with this car, so I'll likely not even be running through a cat.  

I'll get to the head soon, but need to make sure the bottom end will handle things so a new set of rods are likely going to be purchased in a size that eliminates any need to shave the pistons - this will eliminate some costs and strengthen the bottom end.  I'm getting estimates now on a 470g rod which is pretty light compared to stock, matched to the S50 pistons should be a pretty solid weight savings and allow for a pretty high rpm range.

[Boyracer]

Like I said, emissions aren't an issue with this car, so I'll likely not even be running through a cat.

Lucky b*stard!

I'll get to the head soon, but need to make sure the bottom end will handle things so a new set of rods are likely going to be purchased in a size that eliminates any need to shave the pistons - this will eliminate some costs and strengthen the bottom end.  I'm getting estimates now on a 470g rod which is pretty light compared to stock, matched to the S50 pistons should be a pretty solid weight savings and allow for a pretty high rpm range.

Here is Rover K engine of about 1700 cm3 displacement with 32,5 mm inlet valves, forged pistons and steel conrods:



We have larger inlet valves than that, stock! Yummy... Shows what you can do with little displacement, well flowing head and lots of revs

I have thought that pistons are the weak part limiting the revs on bottom end? They are quite heavy and because they are cast, not very strong. Are S50 pistons different in material or construction compared to stock M42 pistons?

I figured that lightweight forged pistons would be a sensible and required upgrade before truly high revs. I think stock M42 conrods are forged so they are quite tough as they are?

[christophbmw]

also, the M44 crank is cast, while the M42 crank it forged and better balanced......dont know if it makes a difference though, they both weigh about the same.