deck height between 2.5 and 2.7?

[rvaughnp]

Can someone state for fact... that there is a difference between these two block or not.
Ran across this in my research for the piston sizing, if there is a difference than i need to know as this would make a big difference when ordering pistons.

Thanks,

Rvaughnp

[jlevie]

The block for an M20B25 and M20B27 is exactly the same. The difference between the two short blocks lies in the crank, rods, and pistons.

[whodwho]

There does seem some discrepancies on this subject, it would be nice if somebody had access to both to confirm.

This Mahle doc floating around claims .5mm between them??? http://flyingbrickperformance.com/Bu...istons_M20.doc

[rvaughnp]

Posted after whodwho's post. I forgot about that page.
And also:
http://www.daham.org/16vt/technical/bmwm20/ (1/2 way down)

As this writeup seems to be knowledgable, i don't want to put out any misinformation in my piston search.

I certainly don't want to go chasing a rabbit either.

rvaughnp

[Brian D]

Are you going to assemble the engine yourself, or do you have an engine builder? Are you going to deck the block, to assure it's flat? I think the figures stated in the Mahle doc are assembly heights, not the actual block dimension-iow, the 206.2 dimension puts the piston .5mm down in the bore.

I would plan for 206 nominal, and shave the block to match. Actually, you should have the block measured first, to make sure the bore will clean up with your chosen overbore.

[rvaughnp]

in my build i will be building the motor. I was planning on getting a chilton of sort and check block throguh those dimensions.

I was not anticapating on decking the block mainly because i have not heard/read where that is a problem with these motors. But i will check it first. Additionally, what i know is the whole timing issue with shaving the block and trying to get the cam to line up properly. I don't want to get into adjustable gears.

To the extent of the 'engineering" side of all this... its a work in progress. I have built many American engines. Mostly all straight foward. No need to delve off into measurement... just order for what you want and have... done. Put it together, turn the key, drive away. This BMW is a bit different.

Any who, after I get my 2.7 motor i can measure it then. Just hoping someone who has physically measured one before, would chime in before i go down that road.

As my m20 block is toast and no need to get one of each if I can help it.

rvaughnp

[kenika65]

all m20 blocks are identical except for the ix block...

[nando]

in my build i will be building the motor. I was planning on getting a chilton of sort and check block throguh those dimensions.

I was not anticapating on decking the block mainly because i have not heard/read where that is a problem with these motors. But i will check it first. Additionally, what i know is the whole timing issue with shaving the block and trying to get the cam to line up properly. I don't want to get into adjustable gears.

To the extent of the 'engineering" side of all this... its a work in progress. I have built many American engines. Mostly all straight foward. No need to delve off into measurement... just order for what you want and have... done. Put it together, turn the key, drive away. This BMW is a bit different.

Any who, after I get my 2.7 motor i can measure it then. Just hoping someone who has physically measured one before, would chime in before i go down that road.

As my m20 block is toast and no need to get one of each if I can help it.

rvaughnp

call metric mechanic and ask them what the deck height is

[rvaughnp]

good idea. i will tomorrow.

Thanks,

rvaughnp

[rvaughnp]

Ok, this post is not meant to discount anyones thought or previous statement. I thought i would get input from someone that pretty much everyone here would respect his opinion.

So...

Here are the words out of Jim Rowe’s mouth. In case you don’t know who Jim Rowe is, he is the owner of Metric Mechanic and builds nothing but BMW motors. He has built hundreds of m20’s.

1)There is no difference in deck height between 2.7 and 2.5 motors, they are both m20 blocks.

2)There are some differences but only in the lower end. In brief… the 2.7 or m20b27 has oil passages to deliver oil to the lower part of the crank bearing. The 2.5 or m20b25 does not have these.

3)The casting is slightly wider in the lower portion of the 2.7/m20b27. He stated he prefers to use this block because there is less material to cut away when installing a large crank and the oil passage.

I know this second party information, until someone can get Jim to join this site and make a reply, you will just have to trust this information to be true.

To the misconception of the differences in “deck height” of 206.7mm for the 2.5 and 206.2mm for the 2.7. Jim believes they are calculating the internal measurements e.g. crank (mm) + rod (mm) + piston height (mm). Which Jim defines as “Stacking height” not actual deck height.

This post is meant as a statement and in no way will I post replies speaking for Jim. We had a 10 minute conversation. This was the topic, the above mentioned was his answer. I am satisfied. Moving on.

Oh, thanks nando for the idea.

rvaughnp

[whodwho]

Excellent, that one has been lingering for a while and now can go to bed :)

Thanks for making the call

[Brian D]

I think the figures stated in the Mahle doc are assembly heights, not the actual block dimension-iow, the 206.2 dimension puts the piston .5mm down in the bore.

To the misconception of the differences in “deck height” of 206.7mm for the 2.5 and 206.2mm for the 2.7. Jim believes they are calculating the internal measurements e.g. crank (mm) + rod (mm) + piston height (mm). Which Jim defines as “Stacking height” not actual deck height.

[rvaughnp]

Thought i would post a pic of a 2.5 with stock piston sticking out of cylinder.


This measurement is approximately 1mm.