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Offset ET28 hole in ET25 for more valve lift

johnmead

Well Known and Active Forum Website User
Non-VOC Member
Has anyone drilled out the rocker pivot hole in the rocker arm and pressed in a bushing with an offset hole to add lift to the valves? I have heads that are good for .500 lift without valve guide to valve stem collar interference.

If so, how much was the offset and what was the increase in lift?

John Mead
 

timetraveller

Well Known and Active Forum Website User
VOC Member
You also have to move/modify the rocker bushes outwards to allow for the offset. Standard lift with MkIIs is about .36" but if you use a standard Vincent cam ratioed up .5" then you will find that the acceleration, jerk and jounce are all above what is considered to be the upper limit.
 

johnmead

Well Known and Active Forum Website User
Non-VOC Member
I am using Terry Prince MK5 cams and radius lifters. I have a set of the long ET26's and plan of moving the locking hole or turning it over and making a new offset locking hole. If that does not work I will machine new ET26s with offset locating holes.

The MK5 cams have 11mm lift on intake and 10.5 on exhaust so I do not have to use much offset to get and additional .050 lift.

John Mead
 

timetraveller

Well Known and Active Forum Website User
VOC Member
OK John, that makes a huge difference. Have you checked what the lift actually is at the valve stem? The reason that I ask is that the ratio of the rocker is not exactly 1:1 over the whole lift. Towards the maximum the outer end of the fork touches, rather than the inner end thus making the ratio better for your purposes. It might be that you are already getting near to the .5" you would like. Once you have all this working several of us would like to know if you can notice the difference. Also what size carbs. you will use to make better use of the increased lift. Cheers.
 

vibrac

Well Known and Active Forum Website User
VOC Member
I had to pack out the valve caps on the thead with thin steel washers to clear the spring retiners and on some Egli-Eglipattern frames you can run out of clearance to the oil tank
 

greg brillus

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VOC Member
I too am using Terry's MK 5 cams and radiused followers. Terry told me it would give 11 mm at the valve, because I am making new valves, I am going to shift the step on the valves up the stem by 80 thou, and lengthen the tip by 1/4 inch. Need to watch that the rocker doesn't top out on the upper guide, but I will probably thin down the collar to help here.
 

roy the mechanic

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VOC Member
The biggest trouble is the clearance between the collar on the vale stem and the upper valve guide. .500" valve lift is very difficult to obtain with standard parts. Greg, I think you may have "cracked it" with special valves, be lucky, and please report back on your findings.
 

timetraveller

Well Known and Active Forum Website User
VOC Member
I have a mock up of this but not used in anger yet. In my case I got valves made with longer thick part of the stems to allow the extra lift. In the heads I tried it on with new valve seats there was just clearance for the fork ends with everything else in place. If it needs a few thou taken out of the lower face of the upper guide in the centre to clear the fork top surface then there is plenty of metal there to play with. If the valve seats were well down then the whole lot will move upwards and cause a problem.
 

johnmead

Well Known and Active Forum Website User
Non-VOC Member
.500 is NOT a problem. If you read my previous emails, I stated that I am using Terry Prince heads which are good for .500 without any modifications to the top of the lower valve guide.

John Mead
 

johnmead

Well Known and Active Forum Website User
Non-VOC Member
I too am using Terry's MK 5 cams and radiused followers. Terry told me it would give 11 mm at the valve, because I am making new valves, I am going to shift the step on the valves up the stem by 80 thou, and lengthen the tip by 1/4 inch. Need to watch that the rocker doesn't top out on the upper guide, but I will probably thin down the collar to help here.
Greg,
You do not need to make new valves. I have made up new EW35s with a top hat shape and a longer body to correct the valve geometry. For valve springs, I am using beehive ones from R&D valve springs in California. They install at the same height as stock Vincent springs and allow .500 lift without binding. They also come with Ti retainers and keepers. I had a machinist cut the keeper groove in each valve using a tool post grinder to replace the Vincent circlip.

Now...has anyone dome what I asked in starting this thread. That is putting a offset bushing into an enlarged rocker pivot hole to give some offset and higher lift and what offset did you use and what was the increase in lift?

John Mead.
 
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