Push rod-tube bores in crank case.

Pushrod Twin

Well Known and Active Forum User
VOC Member
I have read a number of lines over the years which indicate that some Series D cases had the push-rod tube bores inaccurately positioned in the case. KTB, in Chpt 14 "The push-rod Tubes" paragraph states that some were machined "oversize or were not square to the axis of the hole"
I have also read that an indication of this discrepancy is the reluctance of the tube to enter the hole when fitting.
I have a Series C motor with die cast cases. When assembled and run a year ago, the exhaust push-rod tubes leaked at the case. Both front push-rod tubes were reluctant to enter into their respective bores. I recognise that there can be other factors, such as barrel length & head machining which can contribute to the reluctance of a push-rod tube to fit nicely. My barrels are new, standard length muffs with faces square to the bore axis. The front head has an unknown history, the previous owner sourced it from the UK and it had clearly been reclaimed from someones scrap pile. It has ferrous/stainless valve seats, (great) sloppy new bottom guide, & copious weld repairs & re-machined combustion chamber complete with porosities around the cylinder sealing ring. It also had badly worn rocker tunnels which failed to clean up when I bored them over-size. However, in spite of all that, it sealed after lapping & ran satisfactorily. The objective of that explanation was to indicate that the push-rod holes could be anywhere!

I currently have the motor stripped again & would like to find some definitive dimensions to enable me to check the position of all push-rod tube bores in the case. I have the ability to place it on a rotary table in the mill, but need the datums & angles to make measurements from.

Can anyone help?

Thanks in advance, PT.
 

Vincent Brake

Well Known and Active Forum User
VOC Member
Hi PT,
the closed in angle ON THE tubes is 65 degr.
center or interference point of the 2 lines, is about on a circle of 22 mm and the middle centre line of the cylinder (say the base cam circle) (above of course)

well that doesnt help i guess
sorry
 

Pushrod Twin

Well Known and Active Forum User
VOC Member
Thank you Vincent. I have been given a drawing from a local source with some of the answers. It shows the included angle, but with no figure attached. I now know that it is 65 Deg, 32.5 either side of the barrel axis. The point of intersection appears to be on the barrel deck of the crankcase which, I think, is 0.500 from the cam spindle. Can anyone confirm?
A break out on the drawing shows the hole centres from the timing cover split line. Front to back they are; 1.968", 2.156", 0.718", 0.906" with a tolerance of +,- 0.010".
Measuring mine indicates, miraculously, that they are within tolerance, just! Considering that I rubbed the splitline & probably lost 0.002-.005", that indicates to me that perhaps the push rod holes are not so far out after all, unless they are not on the 65 deg lines.
So I fitted the heads & barrels then dropped a piece of 5/8 bright bar down the push-rod holes in the head to see where in the crank case they sat. The picture shows all. Three holes would benefit from moving forward about .030" and maybe a little sideways. The rear inlet is perfect!
Suspecting that maybe the heads are less than perfectly machined, I swapped them for my '48, low miles, only ever raced half a dozen times, B heads. The answer was the same.
Then I fitted the rockers & push-rods to see where they fit. It is quite clear that if I move the holes in the case in the direction they need to go, that clearance between the push-rods & the push-rod tubes with diminish. Ho-hum, where to next???
I think I will split the crank for some rectification there & cogitate a little. :confused:
IMGP4176.JPG
IMGP4176.JPG
 

greg brillus

Well Known and Active Forum User
VOC Member
Roy, you need some of those extra special.......... thin walled......... flexible.......... push rod tubes.................;).
 

Bill Thomas

Well Known and Active Forum User
VOC Member
Hello Roy, Would it help if the heads were 1/2" further away, A better angle ?, You could make up 1/2" spacers under the barrel, And make a pair of Flywheels with 1/2" longer stroke, Maybe 1240cc ?.
And while you are at it, You could make me some Flywheels, With the crankpin 1/4" nearer the rim, For my 560cc Comet ??.
Even if it still leaked oil, It would not matter, It would be SO fast !!.
Cheers Bill.
 

litnman

Well Known and Active Forum User
VOC Member
Terry Prince head.jpg

Terry Prince has used modified VW / Porsche pushrod tubes on his stroked motor. Not an easy mod but it does work.
 

Pushrod Twin

Well Known and Active Forum User
VOC Member
View attachment 14793
Terry Prince has used modified VW / Porsche pushrod tubes on his stroked motor. Not an easy mod but it does work.
Aaahh, those are the tubes Greg was referring too, I thought he was taking the mick! :D
I did think about calling up Dan Smith for the design details of his dog leg tubes, they are so practical, but not visually appealing.;)
Raise the barrels Bill? Good thinking Batman, I will try slipping bits of tool steel under the barrels until we arrive at the best compromise & calculate a stroke length from there. That could fix the front 3, then perhaps a VW gumboot as illustrated above for the currently perfect one? :cool:
 
Top