Looking for certainties: b20 vs b25 cylinder walls

[Nagash]

Some months ago I was looking for info to build the m20b20 to b28 thing (I really thanks reggid for all the useful information).
The only thing I'm worry about now is the water jacket / cilinder thickness in b20 vs b25.

I'll explain:
If I bore out the b20 cylinders from 80 to 84mm, will I get the same cylinder wall thickness of an m20b25?
Does anyone have any numbers about cylinder wall thickness of m20b20 and m20b25?

I have two b20 without head now, I could somehow measure the wall thickness (but I don't know how to do that!)


This is the only thing that stops me from doing the 80 to 84 bore: I don't want to have an unreliable engine due to possible cracks on cylinder walls!

Thanks!


Edit: I find out thanks this link that b25 cylinder wall is about 6.8mm thick, with an oil passage at the bottom that reduce the thickness in that point to 2.4mm.

I would really love to find similar measures for b20 block, If I could find a way to do this kind of measures on my block without cutting it in half would be great and I could share the info

[steve_k]

i can't help with running the numbers (wall thickness etc) but i do remember when i rebuilt my 2.7 i used a b20 block to ensure that i had perfect bores that would last a long while,

this was done by an old skool engineer who also cut "reliefs" into the bottom of the bores if i ever wanted to use a crank with bigger counterweights, he also "de stressed" the block (never heard of that term till them) as well as decking it & O ringing the tops of the bores & installing oil squirters (oil jets that squirt oil onto the underside of the piston to aid cooling).

like i say he was an old skool engine builder.

hopefully for you a more knowledgeable member will se this thread & chip in.

[steve_k]

forgot to add, an ex member on here called ant (formerly A-tech) would bore out a b20 block to 84mm (b25) when he built a 2.7/2.8 for a customer.

[reggid]

you can use an ultrasonic thickness measurment device

otherwise look at the deck and measure thickness



then look w3ith a torch down the hole to see if the thickness is uniform down

also look in through the hole for waterpump and there might be one at the back

this will give you some idea.

its been done many times though

[Nagash]

Thanks for the replies.
I know that has done a lot of time, but I'm still a bit worried about doing that, especially for long-term reliability.
The cost for doing that is also really high in the place where I live, I don't want to re-open the engine or making a new one after let's say 30'000 km.

I managed to approximately (!!! huge approximation !!!) measure one of the cylinder walls and I find out it is ~6mm (considering the error, it could be the same 6.8mm thickness of b25 but on the 80mm bore. But it seems to be even less to me..).
If the measure is "correct", an 84mm b20 will have a cyl wall thickness of about 4mm. I have not idea if it's safe or not

I made this short video that shows how the cylinder wall thickness is not uniform: it starts from ~1.1mm and immediately starts to get thinner.



I will remove the water pump from the block and see if I can make a better measurement from there.
When I'll bring the block to the machine shop I'll also ask to the machine shop to sonic-check the thickness.

[Brianmoooore]

My take on this is that most blocks will be OK in that they fall well within the casting and machining tolerance limits set by BMW back in the day, BUT, some will have castings where the casting mould and/or machining was at, or close to, the limit of the allowed tolerances, and these are the blocks that will give trouble.

[reggid]

thats 11mm and ALOT