Forged Pistons and Rods for M20

[agisthos]

I am about to do what I have wanted to do for a long time, which is pull down the motor and do a complete balanced rebuild. Its going to be expensive and labor intensive, so i may as well get new pistons and the rest while doing it.

I want to know what the deal is with forged pistons and rods for the M20. I have an M20B23, an unusual spec, but it seems JE Pistons do them for this model. Ross Pistons is another I came across.

What are your experiences with JE or any other manufacturer of custom forged pistons for the M20? And rods?

[agisthos]

This is also a street car, not racing, so I presume I need an alloy that does not expand or contract as much. i.e get 4032 and not 2618?

[digger]

JE, CP etc would be a reasonable choice. They should be able to do a 4032 alloy. Lucky the 731 head is meant to be used with flat top so no fancy domes required. I'd upgrade to 135mm rods such as a set from an OE forged 24v m50/s50 and the like since you're getting new pistons . Save your pennies no need for forged aftermarket

[agisthos]

JE, CP etc would be a reasonable choice. They should be able to do a 4032 alloy. Lucky the 731 head is meant to be used with flat top so no fancy domes required. I'd upgrade to 135mm rods such as a set from an OE forged 24v m50/s50 and the like since you're getting new pistons . Save your pennies no need for forged aftermarket

But doesn't the 2.3 used 130mm rods? Or can we still upgrade to 135mm in this case?

[E30-TourZing]

But doesn't the 2.3 used 130mm rods? Or can we still upgrade to 135mm in this case?

It seems the B23 does use 130mm rods, but with custom pistons 135mm should be considered.

Digger and I both use Metric Mechanic products. They are great people to talk to about your build. Bring a note pad when you speak to Jim. He wont stop, and you want to retain everything you hear.

They will recommend coated bearings, and the molly rings, etc. Digger will give you better advice then me about if that is worth it or not. I bought the loot recommended to me, but I have not finished my motor yet, so I cant give testimony.

I tore down a M52B28 and took the rods and crank.

[digger]

But doesn't the 2.3 used 130mm rods? Or can we still upgrade to 135mm in this case?

yes it does, you just reduce the compression height (distance from top edge of piston to centre of pin) of the custom pistons by 5 mm to compensate. The b23 OEM pistons has a really tall compression height which is not good for performance

are you going to retain the b23 crank? what size bore? many in the UK have gone to 84mm with the 80 mm blocks so thats an option.

[digger]

It seems the B23 does use 130mm rods, but with custom pistons 135mm should be considered.

Digger and I both use Metric Mechanic products. They are great people to talk to about your build. Bring a note pad when you speak to Jim. He wont stop, and you want to retain everything you hear.

They will recommend coated bearings, and the molly rings, etc. Digger will give you better advice then me about if that is worth it or not. I bought the loot recommended to me, but I have not finished my motor yet, so I cant give testimony.

I tore down a M52B28 and took the rods and crank.

MM's stuff is not mean for 200/731 head. he needs a flat top and can get cheaper pistons elsewhere TBH.

OE bearings are fine e.g. KS brand etc etc
i'd look to get a conventional total seal ring when ordering. exactly what they have will be determined by what bore size so the bore size should be chosen after seeing what ring options are out there.

[Andre3127]

I hear all over that longer rods are "better" in that they reduce side loading of the piston, but keep in mind that they change rotating assembly geometry and effectively create more dwell time at TDC and BDC, which can affect performance, and increases max piston speed.

Do your research. Short and long rods both have their advantages and disadvantages.

[E30-TourZing]

I am about to do what I have wanted to do for a long time, which is pull down the motor and do a complete balanced rebuild. Its going to be expensive and labor intensive, so i may as well get new pistons and the rest while doing it.

Does it not interest you to get a 885 cylinder head? I would assume M20's are cheap in your country?

[digger]

I hear all over that longer rods are "better" in that they reduce side loading of the piston, but keep in mind that they change rotating assembly geometry and effectively create more dwell time at TDC and BDC, which can affect performance, and increases max piston speed.

Do your research. Short and long rods both have their advantages and disadvantages.

speaking of research...
- longer rods don't dwell more at BDC, its the opposite
- longer rods don't increase max piston speed, its the opposite

while yes the piston does dwell slightly more at TDC but calculate the difference in combustion chamber volume at every degree due to a 5 mm difference and you'll find it might as well be zero.

the short rod pulls harder on the port so with crappy port you'll hurt the VE at the topend more if the cylinder head isn't very good.

[agisthos]

Thanks for all the info a lot to mull over here. The 731 head is in good shape and has a shrick cam already, so I wanted to keep this and just freshen up the bottom end.

But I guess it all comes down to the motor builder once they pull it apart and test everything which direction i can then take.