Big bore

[driftwood]

I see a lot about making stroker engines but nothing about larger bore sizes. How far can a m20b25 be stretched?

[maggspower]

The standard bore on m20b25 is 84mm, a m20b20 can safely be taken out to this and is good for getting nice fresh bores when building a stroker. Even though the bore spacing is the same, the casting of the m20b25 is different and the cylinder walls are said to be thicker.

The theoretical max is 86mm, but personaly I would say 85mm to err on the side of caution and retain block stiffness.

The problem is pistons, custom or from another engine, I'm all ears if you have any suggestions on either Which ever they are going to come with a servere dry bumming on price.

This is why people dont talk about it much, you are entering the game of "deminishing returns". A longer stroke crank is going to cost you £100 be it an Eta or m52, and may net you 15bhp, a set of custom pistons will be nearer £800 and may not get you 10bhp. Its all about what other mods you have to go together as a package.

[HairyScreech]

I think 87mm has been done, however it seems not of much benefit unless the engine needs a rebore anyway.

[reggid]

bigger bore allows bigger valves or atleast less shrouding

[HairyScreech]

Trouble with the m20, as i'm sure you have seen is there isn't a lot of room between the two valves.
The biggest i can find that will fit is a 44.5mm inlet from an Essex. Even then its not a direct fit.
The two realistic options are either 43mm valves from Ireland engineering or a better option is the rec valves 503 andg504. These are 34.5mm and should work with stock seats, the catalogue doesn't show anything other than "bmw" but after speaking to Paul ivy he assures me they are m20 valves, something which the spec backs up.

Someone did also say something about using a chevy valve but i find it odd as i have found nothing even close yet.

[reggid]

There are always custom valves.......$$$$.....not for budget build but an allout motor
You can do a +2 delta (i.e. +1 INT & EXH or +2 INT & Std EXH) relatively safely with a performance cam (approx ~4mm lift at TDC)
I have not looked into the seat details and what size the std seats from 885 can be machined to accommodate.

[HairyScreech]

I'm told they will safely accommodate a 43.5 hence a few places offering that as maximum.

I'm not so sure on increasing the exhaust valve, from everything i have read and seen its a bit large as stock and a 44mm inlet would bring the ratio inline with how most of the top guys build engines in terms of flow/curtain area.
I'm guessing you have come across the same?

[reggid]

i would look to go one size smaller exhaust to fit a larger inlet