Author Topic: oversize valves.  (Read 726 times)

0 Members and 2 Guests are viewing this topic.

Offline mad mike

  • Full Member
  • *
  • Posts: 35
oversize valves.
« on: September 12, 2022, 03:09:53 PM »
Hi gents, ..... Can the 410 cylinder head size valves be fitted into a   "K " series cylinder head   without changing the valve seats to a bigger dia. ?  ?   Thanks in advance for your info. 
BELIEVE NONE OF WHAT YOU HEAR, AND ONLY HALF OF WHAT YOU SEE.

Offline MRieck

  • Really Old Timer ...
  • *******
  • Posts: 10,560
  • Big ideas....
Re: oversize valves.
« Reply #1 on: September 12, 2022, 05:47:22 PM »
Hi gents, ..... Can the 410 cylinder head size valves be fitted into a   "K " series cylinder head   without changing the valve seats to a bigger dia. ?  ?   Thanks in advance for your info.
Yes but there is a problem with stem height..... The F2 valves are shorter to fit the F2 head. There are solutions to that problem as well.
Owner of the "Million Dollar CB"

Offline PeWe

  • Really Old Timer ...
  • *******
  • Posts: 15,535
  • Bike almost back to the 70's 2015
Re: oversize valves.
« Reply #2 on: September 12, 2022, 10:27:45 PM »
I have used stock F2 in valves in a K head.
Valve stem very soft, mushrooming very quick.
2 sets of IN valves in the 80's. Had to grind the top sides of stem to get valves out of the guides. I thought I had got a bad batch back then

After 3rd time 20 years later added lash caps on a new set that also added lenght so stem lenght as a K valve.
One thing is that the lash caps are  soft. 0.25 mm dimple before 10.000 km.
Replaced that head a week ago

That head has a new set of 34mm SS valves I bought from CycleX.  (Backcut done here, so ready to install)
TI retainer too. Just need to replace them. Will check lenght seal-keeper groove.

One more difference is the valve relief for keepers. F2 valves not long.
K valves longer than needed and risk for huge oil consumption with higher lifting cams if not using shorter guides or sunken stock guide where a new  stopper groove is cut.

I have recently whitnessed the result when the keeper groove enter the guide seal. It became worse and worse, finally engine turned to an oilpump.
Got lack of oil on a ride. I checked oil and saw how it disappeared and stopped in time beside the road when oil tank was almost empty.

I called my wife, told her where in the garage I have oil and where I was waiting, 50km from home.
Engine did not smoke oil.

The lenght between seal and groove was 9.6mm, lift at running lash around 9.8mm. Higher rpm made it worse too.

That head got the guides redone and back into business a week ago.
« Last Edit: September 12, 2022, 10:29:43 PM by PeWe »
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline mad mike

  • Full Member
  • *
  • Posts: 35
Re: oversize valves.
« Reply #3 on: September 13, 2022, 12:26:11 AM »
  Gentlemen,  thank you for the help and advice,  ....... so if i use a "K" head its a new set of oversize valves,   think  i'll refurb the 410 head, seems a better option.   thanks again.
BELIEVE NONE OF WHAT YOU HEAR, AND ONLY HALF OF WHAT YOU SEE.

Offline grcamna2

  • Not a
  • Really Old Timer ...
  • *******
  • Posts: 15,093
  • I love to restore & travel. Keep'em Going Strong !
Re: oversize valves.
« Reply #4 on: September 13, 2022, 12:49:17 AM »
  Gentlemen,  thank you for the help and advice,  ....... so if i use a "K" head its a new set of oversize valves,   think  i'll refurb the 410 head, seems a better option.   thanks again.

Will you also be using F2 pistons? They're better for what you'll be doing as they'll match the head/combustion chambers of the 410 head.
75' CB400F/'bunch o' parts' & 81' CB125S modded to a 'CB200S'
  I love the small ones too !
Do your BEST...nobody can take that away from you.