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Oil leaking from 3rd fin up on 850 Mk3 head

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Oil leaking from 3rd fin up on 850 Mk3 head

Posted by mike_steer at July 09. 2018

Coming to the very end of my 850 ES project, started the engine and it has the well documented oil dripping from the 3rd fin up on the head. The head is a RH4 which I know are problamatic. Have read some of the posts here earlier and is it the end of the road for the head.  Didn't seem to be any satisfactory solutions to the problem, unless I have read the negative posts that is.  Haven't stripped the head off as yet it to see what the cause could be. From what I understand its either cracked, there is a hole in the push rod tunnel allowing the oil to escape, or a porous cast.  Any other suggestions  before I strip down, and of course solutions to the problem that are tried and tested.

Re: Oil leaking from 3rd fin up on 850 Mk3 head

Posted by norman_lorton at July 09. 2018

Mike, there might be other's comments, but I will offer two thoughts on what has worked for me. Yes it is the castings being porous, and with time and age the oil finds its way through at the shortest point being adjacent to the pushrod tunnel at the 3rd fin.

When my RH4 head on my Mk3 started weeping from 'that area' several years ago, I had it sent off for cleaning and vacuum sealer impregnation. I don't know exactly what the sealant was but I am guessing it was part of a commercial casting seal process. That head has been free of any leak since.

On another Commando head we found a leak from exactly the same 3rd fin place. I took the head off, cleaned it in solvent, and then used a leak detector kit to precisely identify the leak point with red dye. This worked and showed the spot deep between the fins. I roughed up the area with a long grinding tool and used a 8mm diameter spot of Devcon high temperature filled epoxy. That repair worked for a year at least, and then the bike went elsewhere and I lost track of any further news.

Norm.

Re: Oil leaking from 3rd fin up on 850 Mk3 head

Posted by peter_stowe at July 10. 2018

I pretty much agree with what Norman says above as I tried the same as him on 3 cylinder heads, all of which had a crack from the push rod tube to the 3rd find. I tried having one welded by a respected Norton engineer, but he found the aluminium was so impregnated with oil that it could not be welded. I tried the Devcon, but it failed straight away. I tried having it vacuum sealed impregnation, which also didn't work. All this was over a period of 15 years and that leak was driving me mad, ruining the Commando experience, as oil doesn't so much leak, more like pisses out.

My conclusion is that many 850 heads have this crack, and that the problem is crankcase pressure pushing the oil through that damn crack.

So, I found the best way forward is get rid of that crankcase pressure.

If you are sure that your piston/bores, valves/guides are half decent, then the best, cheapest and easiest thing to do is to fit Mikes Yamaha XS650 read valve in the breather from the timing cover.

With a vacuum in the crankcase, air will be pulled from outside the engine through those cracks into the engine. It has worked perfectly for me for many years and also cures many other oil weeps from the engine, like the tacho drive, for about £25. I also felt a slight increase in engine performance as the pistons are now not desending into a pressurised crankcase. I have two friends who have also done this modification with the same results as me.

 

So, Google Mikes Yamaha XS 650 reed valve, get one, and you can buy me a pint when I see you.

Re: Oil leaking from 3rd fin up on 850 Mk3 head

Posted by Peter Denham at July 10. 2018

Previously peter_stowe wrote:

I pretty much agree with what Norman says above as I tried the same as him on 3 cylinder heads, all of which had a crack from the push rod tube to the 3rd find. I tried having one welded by a respected Norton engineer, but he found the aluminium was so impregnated with oil that it could not be welded. I tried the Devcon, but it failed straight away. I tried having it vacuum sealed impregnation, which also didn't work. All this was over a period of 15 years and that leak was driving me mad, ruining the Commando experience, as oil doesn't so much leak, more like pisses out.

My conclusion is that many 850 heads have this crack, and that the problem is crankcase pressure pushing the oil through that damn crack.

So, I found the best way forward is get rid of that crankcase pressure.

If you are sure that your piston/bores, valves/guides are half decent, then the best, cheapest and easiest thing to do is to fit Mikes Yamaha XS650 read valve in the breather from the timing cover.

With a vacuum in the crankcase, air will be pulled from outside the engine through those cracks into the engine. It has worked perfectly for me for many years and also cures many other oil weeps from the engine, like the tacho drive, for about £25. I also felt a slight increase in engine performance as the pistons are now not desending into a pressurised crankcase. I have two friends who have also done this modification with the same results as me.

 

So, Google Mikes Yamaha XS 650 reed valve, get one, and you can buy me a pint when I see you.

Re: Oil leaking from 3rd fin up on 850 Mk3 head

Posted by Peter Denham at July 10. 2018

This mod worked brilliant on my 850 Mk11a .Still a slight leak Instead of 5.

Re: Oil leaking from 3rd fin up on 850 Mk3 head

Posted by graham_reeves at July 10. 2018

Hi

I recently had some first class laser welding done on my Norton by EMP Tooling in Havant. Whilst I was there they told me they had repaired a number of leaking Norton heads by this technique and I spoke to a well known Norton engineer who said he had used them.

May be worth a try.

 

Graham

Re: Oil leaking from 3rd fin up on 850 Mk3 head

Posted by john_holmes at July 10. 2018

The XS650 valve is really an exhaust recycle valve, plenty going cheap on eBay with varying pipe configurations so pick the one that suits your breather pipe setup.

Search for EGR valve in motorcycle parts on ebay.

Re: Oil leaking from 3rd fin up on 850 Mk3 head

Posted by allan_walker at July 11. 2018

Specifically, a Yamaha 125 EGR valve.

Re: Oil leaking from 3rd fin up on 850 Mk3 head

Posted by richard_payne at July 11. 2018

Third fin up is also where any oil leaking from higher up will appear at the front. This can be invisible as it tracks from the rocker spindle covers down inside the casting...or from right at the back if it is from the inlet rocker cover or even the centre nut.

Somehow it seems much more visible when it gets to the front edge.

My first check would be the rocker spindle covers....especially if you've replaced them and they're stainless as they are quite often not truly flat.

RH4 heads don't have to break through / crack. Mine's had several changes of guides etc. and was dry last time I looked...but I have had the odd panic with covers or end plates...

Re: Oil leaking from 3rd fin up on 850 Mk3 head

Posted by Martin Peacock at August 11. 2018

Thank you for the top tip.  My MK III 850 had developed some slight weepage from the lower fins on the right side at the front.  I fitted the Yam 125 reed valve in the breather line as suggested and it worked like a charm.  No more weepage even after a 300 mile run.

Re: Oil leaking from 3rd fin up on 850 Mk3 head

Posted by ashley_cutler at August 11. 2018

Check the top of the nuts in the cylinder head in front of the centre bolt. You will need a little torch to peer in between the fins, you may find the oil leaking up the stud.

Re: Oil leaking from 3rd fin up on 850 Mk3 head

Posted by alan_elstob at August 11. 2018

Another way of checking where the oil is coming from if you are unsure is to tamp some cotton wool in between the third and fourth fin, having cleaned up the head beforehand. Then go and do 100 miles. On my machine the oil leaking from the crack between the fins did not start to penetrate through the cotton wool for at least this distance. It was clear then where the oil was coming from.

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