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Post by Fox on Dec 3, 2014 1:43:58 GMT -5
But I put longer valves in a shorter valve head, so that would give them more preload, correct? Not if the platform the spring sits on is machined deeper in the head. The longer valve would move the valve keeper farther away so you don't have to compress the spring as far to install the keepers which equals less tension/preload |
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Post by Clank on Dec 3, 2014 1:47:49 GMT -5
But I put longer valves in a shorter valve head, so that would give them more preload, correct? Not if the platform the spring sits on is machined deeper in the head. The longer valve would move the valve keeper farther away so you don't have to compress the spring as far to install the keepers which equals less tension/preload So couldn't I remedie that by placing washers under the springs? |
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Post by Clank on Dec 3, 2014 1:51:44 GMT -5
Also when I did swap them they both sat at the same height as they where in their original heads. In other words the 69mm valves sat about 3/4" above the top of the head in both heads.
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Post by Fox on Dec 3, 2014 2:06:08 GMT -5
well that's the crucial measurement I'm talking about. The height of the end of the valve measured from the spring platform. The stock head looks like it has a much shallower dome than the BBK head. If the dome difference moves the 69mm valve seats 5mm's farther away from the platform then they should work fine but looking at this pic of a 69mm head for sale on ebay the springs look more compressed than yours.  |
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Post by Clank on Dec 3, 2014 2:09:38 GMT -5
So would it hurt anything if I put a washer under each spring?
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Post by Fox on Dec 3, 2014 2:14:08 GMT -5
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Post by Clank on Dec 3, 2014 2:33:38 GMT -5
There must be a misunderstanding, I have 3 69mm heads(head, valves, springs, and rocker assemblies) and one 64mm bbk head. The bbk head with the 64mm was said to have been "professionally" ported and polished. But who ever did the p&p didn't take the values out and the had grinder knicks in them. 190mech suggested that would create a weak spot(which I agree) and recommended replacing them. Well all I had was 69mm valves so I put one of those in the bbk head. So I guess I'm gonna pull the head when weather permits and put the stock 69mm valves back in the stock 69mm head and use the stock head which is 69mm. :confused: |
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Post by jhobe6678 on Dec 3, 2014 5:32:09 GMT -5
I was just going to say why not just go back to the stock head? The only benefits of a BBK head or any other head (shut up you perverts!!!) is if the valves are bigger in diameter. I.E. stock is what, 17mm intake 15mm exhaust? So if you had one with valves that were 19mm intake and 17mm exhaust. Valve length has no direct affect on performance, but shorter valves seem more "stable" to me. As long as the valves and rocker arms match, the head has no bearing on what size go in there. I was going to order the BBK head from John and he asked me did I want 64mm or 69mm valves. I asked what the difference was and basically got "one is 64mm, one is 69mm... " lol He basically said find out what size I have no so I can just use the rocker assembly from it.
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Post by Fox on Dec 3, 2014 15:16:44 GMT -5
Since the dome on the BBK head is bigger around and higher than the stock head I would venture to say that the compression would be higher using the stock head. Less volume = higher compression. That's simple math. I can't say for sure but I think there is such a thing as too much compression but all that scientific stuff is beyond my pay grade.
The manual specs say the free spring length should be 30.5mm +/- .2mm That's uninstalled. Just a spring laying on the bench. It also says that if they are less than 28.5mm replace them because they won't have the proper tension. There is no spec on the spring length installed so we get back to the dome height and the amount of valve stem exposed above the spring platform to determine whether or not the springs have the proper amount of pre-load. Logic would dictate to me that a longer valve in a short valve head = less spring tension.
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Post by Clank on Dec 3, 2014 18:14:39 GMT -5
Thank you Fox and jhobe6678. I guess when weather permits I'm gonna swap the heads. That great info Fox, where did you find it? :thumb: |
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Post by Fox on Dec 3, 2014 18:38:44 GMT -5
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Post by Clank on Dec 4, 2014 18:58:27 GMT -5
You were right about spring platform Fox.    the one on the left in the first 2 pics is the 64mm BBK head with 69mm valves and the one on the right is a stock 69mm head and valves. It's reversed in the last photo. Good call on spring preload, that completely slipped my mind. But now we all know. :thumb: |
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Post by FrankenMech on Dec 4, 2014 19:35:48 GMT -5
That is a good use for those large aluminum pans from the grocery store.
Notice how much tighter the springs are compressed in the top right photo. There must be a spec on assembled spring height somewhere.
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Post by Fox on Dec 4, 2014 19:42:14 GMT -5
Yeah there's the pics I wanted to see. Glad I could help. I wasn't 100% sure I was correct cause I haven't looked at both head types side by side like that for quite some time.
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Post by Clank on Dec 5, 2014 0:01:27 GMT -5
That is a good use for those large aluminum pans from the grocery store. Notice how much tighter the springs are compressed in the top right photo. There must be a spec on assembled spring height somewhere. Lord forgive me for saying this. You can get 2 for 89cents at Wally World. |
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