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Author Topic: Return oil issue with cylinder heavy studs  (Read 461 times)

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Offline RegDeg

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Return oil issue with cylinder heavy studs
« on: December 02, 2019, 02:29:48 am »
Hello guys!
I’ve rebuild for a customer its K0 Cb750 engine and, to avoid oil leaks, i’ve put in new cylinder heavy studs with MLS head gasket : no retorquing after 1000kms needed I thought...
After this rebuild i’ve made a test bench to start the engine without putting it in the frame. Good idea because I’ve seen lot of smoke out of the exhaust.
I’ve made 2 others starts after cooling, thinking the mounting oil in pistons, valve guides, etc need to goes. Same smoke.
Just after the last start i’ve unscrewed the exhaust valve taps and seen lot of oil around the spring, up to the retainers!
This morning  I’ve dismantled all the cylinder head and ask you if someone have already seen this?
I’m a little afraid because it is the 1st time I see that while I’ve already install this stuff on 2 other engines without seeing this smoke.
I think there is an issue between the head/ cylinder block/ MLS gasket holes diameter and the heavy studs diameter. Too much oil comes in the head and the difference diameters are too little to permit the oil going down in the case by the studs way.
Does somebody have already seen this and what could be the solution? Reinstall stock studs is not the better ! ;-)
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Offline FrodeF2

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Re: Return oil issue with cylinder heavy studs
« Reply #1 on: December 02, 2019, 05:14:01 am »
Are the oil restrictor jets installed in the head?
Just thinking it could be the supply to the head that is the problem, and not the return.

Frode j


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Offline RegDeg

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Re: Return oil issue with cylinder heavy studs
« Reply #2 on: December 02, 2019, 05:32:10 am »
Yes, as it is a K0 head, they are machined in.
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Offline PeWe

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Re: Return oil issue with cylinder heavy studs
« Reply #3 on: December 02, 2019, 11:07:40 am »
Interesting.  Good input.

I measured my MLS gaskets and a new cometic fiber. The fiber gasket has +1 mm return holes.

I did not see this with 836 and MLS. Now 72mm MLS, 71.25mm pistons and billet block. No smoking. Kibblewhite studs rhat look like APE hd studs.

But, I had 900 kit with MLS a short while that smoked which I could not find out why. I checked, bores, pistons, all rings etc.  I suspected the gasket and surface.


- What next?
Drill the oil return holes bigger?
Only the MLS gaskets 8 holes?
I think + 1.0mm might be enough.
How to drill them without damage rhe gasket?
Grind with conical stone from both sides?

MLS can be reused.
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Offline turboguzzi

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Re: Return oil issue with cylinder heavy studs
« Reply #4 on: December 02, 2019, 11:41:22 am »
interesting problem... the gap with the mls looks good enough for me, but what about the paper gasket at the bottom?

had to open up once an MLS for a bigger bore, did it with a dremel stone, no issues, just make sure you clean the dust between the mls sheets after

if you want to get an idea of the oil flow, you could easily simulate with a tube with the right diameter hole inside and a metal bar like the stud in the middle, then just pour some oil into it and see if it goes down fast.

when warm oil is like water, so looks a problem only when cold

Offline RegDeg

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Re: Return oil issue with cylinder heavy studs
« Reply #5 on: December 02, 2019, 12:46:34 pm »
On this K0 cylinder, the holes are only at the head side surface : at the bottom there is no holes but a big tunnel and the paper gasket have the same opening so no problem with that. The 2 other engines i’ve done doesn’t seem to smoke, I think it is proper at this engine.
So fortunately the engine wasn’t in its frame but this issue is near to appear with heavy studs installation : we must pay attention at this point.
That seems incredible that little holes jets have sufficient debit to fill up the valves housing!
I’m agree with turboguzzi to say maybe it due to the oil temp, that should be less impacting with hot oil but I use 20w50 and the engine had 2 hot runnings and smoke all the time.

What next? Good question...
Opening holes in the MLS is not a big problem. I’ve done it to adapt to the lightly excentred studs. Just pay attention effectively with metal burr and dust and don’t exceed limit walls. 
For the head and cylinder, I don’t know if I can drill lightly bigger holes, in the head at least up to the connection between holes stud and return oil holes. For the cylinder the holes have maybe 2cms length. Does it modify the strength of the cylinder at these points? I think not but...

PeWe: are you sure I can reuse the MLS gasket? It has 2 hot starts and I’ve read it is not desirable to reuse it after that. Have you already done it? This engine is not a tuned one, just genuine assembly with good studs and MLS to avoid leaks...

Thanks for your response! And excuse my French English... 
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Offline PeWe

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Re: Return oil issue with cylinder heavy studs
« Reply #6 on: December 02, 2019, 01:17:55 pm »
I have reused MLS that had done around 500-1000km. Edit: it must have been much longer use.
 As long as the viton rubber is still intact, no problems.

It is possible to brush all loose surface off and spray with Loctite gasket spray. I have not used that spray yet, a shop nearby recommended that.

Many guys here must have installed many MLS and HD studs.

Hopefully guys with similar experience of oil returns and much oil at top.
« Last Edit: December 03, 2019, 08:12:26 am by PeWe »
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Offline turboguzzi

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Re: Return oil issue with cylinder heavy studs
« Reply #7 on: December 02, 2019, 03:45:54 pm »
i am not a 750 expert by any means... but, now that i think of it, why dont you assemble back the head and try pouring oil into the inlet springs cavities (thats where it would be sucking oil) and see how fast it drains

still, considering it should have good guides and valve seals, hard to see why it would smoke so badly just form oil passing the guides....

does one piston show more oil burnning than others? maybe you broke a ring there or it's stuck...

Offline BPellerine

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Re: Return oil issue with cylinder heavy studs
« Reply #8 on: December 03, 2019, 01:24:20 pm »
is any oil getting out from under the cam towers?member disco had this same problem with his bike,it was not the studs,maybe check with him.bill
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Offline MRieck

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Re: Return oil issue with cylinder heavy studs
« Reply #9 on: December 03, 2019, 07:03:51 pm »
I have reused MLS that had done around 500-1000km. Edit: it must have been much longer use.
 As long as the viton rubber is still intact, no problems.

It is possible to brush all loose surface off and spray with Loctite gasket spray. I have not used that spray yet, a shop nearby recommended that.

Many guys here must have installed many MLS and HD studs.

Hopefully guys with similar experience of oil returns and much oil at top.
I haven't experienced any problems with that combo and believe me.....if AndyCB750's engine isn't leaking after the last top end build you know it works OK. ::) ;D ;D
Owner of the "Million Dollar CB"

Offline RegDeg

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Re: Return oil issue with cylinder heavy studs
« Reply #10 on: December 04, 2019, 02:06:58 am »
The leakage is really due to the valve guides immersion: as it is a K0 head with low km the guides and valves were really ok and didn’t need to be changed. But the exhaust ones are without oil seals.
 So, i’ve rewatched the assembly: the studs are 8mm diameter (7,87 exactly), the head holes are 11,2mm as the cylinder ones but the MLS has 9,5mm dia. The retourne hole between the valve is 6,8mm dia.
I’ve done a little calculation:
9,5dia surface - 7,87dia surface = 70,88mm2-24,32mm2 = 46,56mm2
6,8dia surface =36,31mm2
So the oil passage at the cylinder head gasket is approx 25% bigger than the return oil hole near of the valve.
 By the way this not the real explanation...

Seeing my cylinder i’ve understand the problem.
The cylinder holes are not centered with the studs. I thought it was due to the thread in the case but not. It’s due to the holes which are not centered !!!
See photo for better explanation.
The studs and the gasket holes (already maximised to 11mm instead of 9,5mm dia) are centered together but not with the cylinder holes... so with 9,5 dia in the gasket the debit is minimized and should make the deal....

I think on the later cylinders the problem doesn’t exist with a better hole centering...

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Offline PeWe

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Re: Return oil issue with cylinder heavy studs
« Reply #11 on: December 04, 2019, 03:42:38 am »
Is there a problem deeper down?
Where cylinder mate the case.

Interesting to know if adjusting the gasket oil holes will help.
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Offline RegDeg

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Re: Return oil issue with cylinder heavy studs
« Reply #12 on: December 04, 2019, 08:12:57 am »
Deeper in the cylinder the holes joints in a sort of tunnel keeping the studs together : really big area where the oil can keep flowing.
I will overbore the gasket holes tomorrow and re-install the MLS. Thanks for the information about keeping the gasket. The Viton is still on it, i’ve Just dismantled the 3 sheets/layers  to wash them. I’ve got the Loctite gasket spray red, I will see if I use it on the layers.
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