enlarging bore m30b30

[budgerian]

Is it possible to enlarge the bore of an m30b30 (3.0)
from 89 to 92mm (effectively making it a 3.5, when combined with an 86mm crank) or do m30b34 and 35 blocks have a different cilinder casting (providing more "meat" to the cilinder walls.)

Reason for asking is that the m30b34 block I have needs oversize pistons which are unfortunately made from unobtanium, but I have two m30b30 blocks in good condition, to which I could fit the pistons and crank from the 3.5. I am good buddies with a machinist so the boring costs are not a problem.

Could those answering please focus on the question at hand and not on the why or better this or that, just: can it be safely done?

Thanks for your advice,

Roger

 

[rsporsche]

why not talk to a major engine builder - like Korman (and others), and enquire about Mahle pistons to fit your block.

 

[x_atlas0]

They didn't change the bore spacing, and it isn't a wet-lined engine, so all the blocks should be "boreable" to the same maximum of ~94mm.

 

[roundel]

M30 block

It is not safe to bore an old (E3, E9, E12 etc....) 3;0 litre block to 92mm. The 3;4 and 3;5 blocks are different, thicker cylinder walls & no waterways between cylinders.
Alex

 

[M5dCS]

m30 blocks

Bore spacing is the same for all of the 6 cylinder engines of ther era but the blocks with 3.3 or 3.5 cast into the left side with raised letters have more space around the crankshaft area and therefore do not have interferance problems with connecting rod clearance. Mahle pistons are great but darned expensive, I have used pistons for Toyota pickups with the R22 engine, they have a big compression bump on top that can be machined off to give you the compression ratio that you want, plus they were a little lighter and a whole lot cheaper. I even built a 88mm stroker 02 engine with these and ran it hard for about 150k miles with no problem. They can be purchased only in sets of 4 from Silvalite and they are also available in oversizes. Piston pin size is the same as BMW. Good luck, Leroy

 

[budgerian]

"It is not safe to bore an old (E3, E9, E12 etc....) 3;0 litre block to 92mm. The 3;4 and 3;5 blocks are different, thicker cylinder walls & no waterways between cylinders."

"They didn't change the bore spacing, and it isn't a wet-lined engine, so all the blocks should be "boreable" to the same maximum of ~94mm."

I also thought (and hoped for) the last, but Roundels comments worry me.
One of the b30 blocks I have checked does not have waterways between the cilinders (It's a 3.0/3.2 casting).
Are you sure that's not just with the 2.5/2.8 blocks?

Nevertheless I can see through one of the waterholes on the deck that the outside of the cilinder wall becomes thinner about half an inch down from the deck which makes me wonder if it is indeed not possible.

Who knows for sure and can supply facts?
I need to ask since I need to make an expensive decision based on whether it is possible or not.

 

[roundel]

M30

The bore spacing has nothing to do with it. All of the old M30 up to 3;3 litre blocks had waterways between the cylinders. When the 3;5 litre ,84mm crank and 94mm bore, engine was introduced around 1978 the block was changed and no longer had waterways between the cylinders. When the 3;5 litre engine became a 3;4 litre engine in 1981 with 86mm crank 92mm bore the block still had no waterways.
If you bore an old pre 1980 89mm bore block to 92mm there is a 50% chance you will go into the water jacket. This might not be obvious until you run the engine.
Alex

 

[x_atlas0]

The bore spacing has nothing to do with it. All of the old M30 up to 3;3 litre blocks had waterways between the cylinders. When the 3;5 litre ,84mm crank and 94mm bore, engine was introduced around 1978 the block was changed and no longer had waterways between the cylinders. When the 3;5 litre engine became a 3;4 litre engine in 1981 with 86mm crank 92mm bore the block still had no waterways.
If you bore an old pre 1980 89mm bore block to 92mm there is a 50% chance you will go into the water jacket. This might not be obvious until you run the engine.
Alex

Good to know. I thought they never had waterways between cylinders.

 

[roundel]

M30

If you look at the space between the cylinders on a 94mm bore engine it is plain to see there is no space for a waterway. And if you look on an 89mm bore block you can also see in order to have water flowing between the cylinders there can`t be much thickness to the cylinder walls.
Alex

 

[budgerian]

Alex,

As much sense as your explanation makes, and I do believe that a rebore to 3.5 is out of the question, I can not move anything as thin as a strip of paper between the piston bores (I removed on of the freeze plugs for this purpose). There seems to be no waterway between them.

Could it be that the blocks with 3.0/3.3 cast in the side are different from the early 3.0 blocks which have no engine size casting mark?

 

[roundel]

M30

Do you know what date your block is. it is possible the later 3 litre engines were the same as the 3;4 & 3.5. But certainly the first block without waterways was the first 635 around 1978.
Does you block have reliefs for the con-rod bolts in the block ? As needed for 86mm crank ?

alex

 

[budgerian]

The engine came out of an early (D-jet) E3, but my guess is it's an exchanged unit, for two reasons:
there is no engine no. stamped on the starter ear, just two "X"s (BMW painted a notification on the rear of the block above the flywheel, if it was a replacement engine, but this marking may be long gone..)
second it is marked 3.0/3.3 which indicates an L-jetronic block, as there was no 3.3 in the early days of the E3. It also uses the bigger dipstick like the 3.5 block, whereas the original 3.0 I have uses a much smaller dipstick.
The crank and pan are still on the car so I can not yet check for any difference in the crank area. I will report as soon as the pan comes off.

As I revised my plan into buying oversize pistons for the 3.0 block, please check the new thread I will open about something curious I came up against while looking for suitable offers.

 

[M5dCS]

boring 3.3

I personally bored a 3.3 block to 92mm and used a crankshaft from a 3.5 that I had laying around. The 3.3 marked block does have the reliefs for the con rods and boring it was no problem.

 

[budgerian]

So there is some hope left?
Are these the reliefs I'm looking for?

 

[roundel]

M30

I had a customer back in the 1980`s that bored an 89mm block to 92mm & it was Ok for a couple of months, then the oil started to get contaminated with water. We eventually found a porous bore that was allowing water under pressuire to drip into the sump.
I don`t understand why you would want to take this risk when you can buy a 3;5 litre block for next to nothing ?
Alex

 

[budgerian]

Ah, the dreaded why question, still we managed to get to 14 messages before it cropped up :-D

Because i am not made of money would be the short answer.
Because I have two 3.0 blocks lying around and because with every used 3.5 engine I buy I run the same risk of needing an overbore, thus requiring unobtainable pistons, as i have run into with this engine. This one even ran when I bought it, and it still turned out to be a dud.

In this light combining blocks and internals from two engines would make perfect sense - if only safely doable- which was the original question.

Guess I will check the 3.0/3.3 block for the notches in the casting and then figure out a way to determine cilinder wall thickness.
But even then, theres always the risk of damage/pitting to the waterside of the cylinderwalls due to neglected antifreeze changes, ending in problems like you just described.
Tough call, I realy liked the 3.5 idea...:cry:

 

[M5dCS]

you've got it

Yes that's the right block with the right notches, stick in the boring bar and hog it out, you can do expensive ultra sounds and all of that, but generally speaking, german engine blocks don't have much core shift during casting. Also don't take too big of bites with the boring bar as these blocks are tough(high quality cast iron).

 

[roundel]

M30

My advice would be that if you are 100% sure there is no space between the cylinders then it is probably OK to bore it. If there is space between the cylinders I think it is too risky. Assuming you are trying to do it on a budget, I would think the last thing you want is for it to go wrong after all the effort.

Alex

 

[budgerian]

I appreciate your solid advice Alex.
I am hunting down oversize pistons for the 89mm bore as indicated in the other thread. They seem to be available for a reasonable price.
You are absolutely right in that I want to do this only once..

 

[2275xxx]

Alex,

BMW Motorsport, Schnitzer and Alpina were racing engines with a 94mm bore from March-April 1973 (94x80, then 94x84).

Do you know how they managed to bore the blocks to 94mm at that time ?

Was a special run of blocks cast for the race engines ?
Or did they just bore many stock blocks until a few surviving ones could be selected ?

On the race tracks they seem to have had more problems with cranks and head gaskets than with the blocks.

Marc



Edit: actually Schnitzer and Alpina were boring only to 93.6 on their 3.3 (3303ccm) liter engines, while Motorsport engines were 3331ccm (94x80). Later in 1973 almost everybody raced 3,5 liter engines with 94x84.