Main bearing bores

[Erik in PA]

Hi all,

Probably a stupid question, but I'll ask anyway...

I am preparing to put my 66 912 motor back together
and re-measured my bearing bores as per the Maestro's
literature directions.

Essentially, I took the two case halves (cleaned and
bare) and put them together.

When I took the first measurements I only had the 6
acorn nut case bolts tightened up to 29 lbs. I used
the Maestro's technique of starting at 15 lbs and then
working up to the final 29 lbs.

I measured and found my #3 bore to be 60.20mm, but the
#1 and #2 were 60.30mm.
I began to get very worried since I have all the new
parts ready to go in!

I reasoned that since the case is aluminum, it will
squish more with the perimeter bolts on and torqued.
I installed all the perimeter bolts and cam nuts and
torqued them to 17 lbs.

I re-measured and all the bearing bores then came in
at 60.20mm.

But isn't this too tight?

The Maestro's books (I've got quadruple redundancy)
state that the standard case should have a bore of
60.24mm +/- .005mm.

Am I splitting hairs, do I have a major problem, or is
this case one of the proverbial "tight as a bull in
runtime"?

Thanks,
Erik.

[ScotMcLean]

I just saw the response to your inquiry come across the 356Talk email - measurement sounds right (so look for what's wrong ).

I'll be doing the same soon and would love to see pictures of the process (measuring, assembling, checking measurements again), if you took them. I'm sure many of us have either done this, read about it, or will need to know more about it in the future and would find it interesting! I have Pellow's books as well and while they are great on detail, more pictures would be helpful. Thanks -

Scott
69 912 #3078
67 912 parts car - need parts? (with an engine that will soon be disassembled & put back together)

[mein12]

Hi Erik,
Sounds like your right on. I have assembled engines with less than perfect dims. You'll be fine. JMB

[Roland Kunz]

Hello

I think one digit is shiftet. I remeber 60, 000 to 60,020 with 0,030 as final wear area.

Now it looks like you hace the first overbore with is plus 0,25 mm

The tolerances where then 6,250 to 6,270

The 60,300 would be on the border.

Mostly the bores get slight oval over the years and the middle bearing is the first to show signs for that.

So you have to mesure crosswise to get a middle messure field.

Just look at your books what numbers are the correct numbers as I only know the "fields".

If you go to make a normal engine the tolerances are OK but if you bild up a tuning engine or will go to race then the case needs a new aligne bore up to 60,500.

Grüsse