So I am sure many of you have seen the article from BDK here http://www.bdkraceeng.co.uk/Bike1.html#Ignition
that states:
"If you measure csa(cross sectional area) of oil loss at rod ie:area of pin minus area of rod inc shells, times 2 for both sides . Plus oil loss for outer main brg(since these are fed together). This is the total area for oil loss assuming the crank is not moving(no centrifugal pull). A rule of thumb is that the supply csa is at least a third larger than the loss csa. This also assumes no 90 degree bends etc. If you look at the oil feed in the btm case it has an arduous path to the outer main and consequently the big end. The oil rises in a 5mm drilling (where the restrictor jets are) and then horizontally in a 6mm hole and then up again at 6mm to the outer big end. The 5mm drilling has to be because they wanted to put a M6 thread in it for the jet. Anyway, take the 5mm hole out to 6mm and then use a thread repair, Time sert or Helicoil, to restore the thread for the oil jet. The reason the main is OK is because it always has sufficient oil and little centrifugal force, however the big end throws the oil away at a considerable rate .Remove more than you are putting in and bang! I have seen hundreds of FZR's with outer b.ends gone. All modified as above sorted them out without fail. "


The procedure as recommended is correct but some simple errors make it a little difficult to follow. For clarity here are my corrections based on my FZR400RR 3TJ1. The feed to the outer mains is not 6mm. The final hole is 6mm but there is a reamed section below that (shown in red on image) that is 5mm.

First the corrected text:
The oil rises in a 5mm drilling for main bearings 2 & 5, where the restrictor jets are (2.55mm diameter), and it also branches horizontally in a 6mm hole and then up again at 5mm to the outer main and big end. The 5mm drilling has to be because they wanted to put an M6 thread in it for the restrictor. Anyway, take the 5mm hole out to 6mm all the way to the bottom and then use a thread repair, Timesert or Helicoil, to restore the thread for the restrictor.

The 5mm gallery (shown by blue arrows) is not large enough to feed both the 5mm main & big-end, and the 2.55mm restrictor.




Mains 2 & 5 have to have restrictors because they are mains only and do not feed big ends.



Here is a 5mm drill bit inserted in outer main. It is a very snug fit. Mains 3 & 4, which also feed big-ends are also 5mm or slightly under.


Of course you cannot just enlarge the lower part of that gallery, you have to run a 6mm drill all the way down.

Interesting stuff, there isn't actually any numbers to go with the maths other than to say the 5mm gallery isn't big enough to supply enough oil.

I wonder if opening the holes up to 5.5mm would increase the flow enough to make the difference while leaving the 6mm threads of the 2.5mm inserts intact so avoiding all the helicoil malarky?

If you put a dowel through the gallery to the oil pressure switch you wont drop through and hit the bottom of it when the drill breaks through into it.

Robotnik wrote:Interesting stuff, there isn't actually any numbers to go with the maths other than to say the 5mm gallery isn't big enough to supply enough oil.

I wonder if opening the holes up to 5.5mm would increase the flow enough to make the difference while leaving the 6mm threads of the 2.5mm inserts intact so avoiding all the helicoil malarky?

It is a function of area and 5.5mm is not enough. The helicoil is simple enough.
Diameter Area
5mm = 19.63
2.55mm = 5.11
Total needed 24.74
6mm = 28.27

Thanks for the info, I've heard about this modification but your description and pictures have made it clear to me what needs to be done and why.

Makes you wonder how Yamaha could have made an oversight like this.

From what I know the T-cat crankcases are pretty much identical, I wonder if they have the offending gallerys drilled out to a larger size, or maybe a higher capacity oil pump, do you have any information on that?

Top Tip about the dowel to stop the drill going through the bottom gallery too Evil.

Actually it was Dale - who's modding a 2TK that sent me pictures of him doing this mod that gave me the idea

Hi all,
Based on the fact, that my English is bad, can I repeat the procedure as I understand it.
And please, can anybody of you confirm it.

You remove the restrictors of main bearing 2 and 5 and enlarge the diameter of the bore hole from 5 to 6 mm.
To ensure not to hit the bottom, you remove the oil pressure button and insert a dowel or something similar.
After drilling you have to insert a helicoil to refit the restrictors.
Is this a correct summery?

THX

Hi again,
How important is this change?
I have 3 pieces of 3 TJ engines in use, and no of them has this change.
THX

You need to do it to race and track bikes but not really to a road bike

enduro250 wrote:Hi all,
Based on the fact, that my English is bad, can I repeat the procedure as I understand it.
And please, can anybody of you confirm it.

You remove the restrictors of main bearing 2 and 5 and enlarge the diameter of the bore hole from 5 to 6 mm.
To ensure not to hit the bottom, you remove the oil pressure button and insert a dowel or something similar.
After drilling you have to insert a helicoil to refit the restrictors.
Is this a correct summery?

THX

This is correct.