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Post by Clank on Dec 2, 2014 18:55:29 GMT -5
Ok I've searched all over the forum and the net, does anyone know what the difference is in a 69mm & a 64mm rocker assembly. I've found zip for specs.
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Post by Fox on Dec 2, 2014 20:56:34 GMT -5
The 139QMB's have two heads that are used on them with two different rockers. One head has 64mm valves and the other 69mm. If you install a rocker from a 69mm head on a 64mm head it won't work and vice-versa. I'm not sure if the cam bearing cradle is higher on a 69mm or the whole head is taller but I ran into this issue once when I ordered a standard rocker assy. for a tall valve head. I ended up going to BMS's world headquarters myself and talk to their in-house mechanic who found the right one for me. That guy saved my ass and I learned about the two heads that day. You will have to remove a valve and measure it to know for sure if you aren't lucky enough to have both types of heads present. Scrappy sells both rockers. www.scrappydogscooters.com/Replacement_Parts.html |
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Post by Clank on Dec 2, 2014 21:36:31 GMT -5
The 139QMB's have two heads that are used on them with two different rockers. One head has 64mm valves and the other 69mm. If you install a rocker from a 69mm head on a 64mm head it won't work and vice-versa. I'm not sure if the cam bearing cradle is higher on a 69mm or the whole head is taller but I ran into this issue once when I ordered a standard rocker assy. for a tall valve head. I ended up going to BMS's world headquarters myself and talk to their in-house mechanic who found the right one for me. That guy saved my ass and I learned about the two heads that day. You will have to remove a valve and measure it to know for sure if you aren't lucky enough to have both types of heads present. Scrappy sells both rockers. www.scrappydogscooters.com/Replacement_Parts.html So how bad is it that I took 64 valves out and put 69 valves and rocker assembly in?? The motor seems to run fine, is this gonna cause some major problem??? |
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Post by Fox on Dec 2, 2014 22:21:33 GMT -5
Other than an increase in spring tension I can't see an issue if it's running fine but I have seen broken springs before. Did it seem harder than usual to install the keepers?
EDIT: After looking at scrappy's site again I see now that the taller valves protrude above the valve cover gasket surface where the shorter ones don't and the rocker assy. for the longer valves has a different shape casting than the standard. If I remember correctly the head I worked on, the springs are the same length but the place on the head they sit on is machined higher so I think you may have a softer spring, not a stiffer one after all but again, it's hard to say unless you have both heads in front of you.
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Post by Clank on Dec 2, 2014 23:12:47 GMT -5
  This is them. |
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Post by Fox on Dec 2, 2014 23:40:00 GMT -5
Well looking at them both I see that the deck height is the same on the cam cradles and the gasket surface so if the spot on the head where the springs sit on it is higher on the long valve head then I would think the spring tension would be softer if you install long valves in a standard head if they work at all. That could cause an issue at higher RPM. The dome is much larger on one of those heads I notice.
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Post by Clank on Dec 3, 2014 0:22:23 GMT -5
The one on the left with the bigger dome came with the bbk. The right was stock with 69mm valves. Everyone said I couldn't run 64mm valves with a 69 rocker as the bbk didn't come with a rocker assembly. So I took the stock valves and put them in the bbk head. The springs where exactly the same in both heads. I'm just wondering if this will cause an issue that I am unaware of.. Thanks for the input Fox. Do you have any other thoughts that come to mind? |
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Post by Fox on Dec 3, 2014 0:41:33 GMT -5
So shorter valves in a 69mm head right? That would mean the springs are compressed more which shouldn't make any difference. Might even run better but I have seen broken springs before. Should be okay. Time will tell. You know the old saying: "If it works..."
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Post by Clank on Dec 3, 2014 0:48:28 GMT -5
Other way around. I put the 69's in the 64 head.
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Post by FrankenMech on Dec 3, 2014 1:11:02 GMT -5
The cam bearing cradle height has to be the same on both heads or the chain would have to be a different length. The sprocket ratio must be the same 2:1 on both engines.
Is there a spec anywhere on the valve assembled height?
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Post by Fox on Dec 3, 2014 1:29:27 GMT -5
The cam bearing cradle height has to be the same on both heads or the chain would have to be a different length. The sprocket ratio must be the same 2:1 on both engines. Right. I wish clank had taken pics looking down on the tops of both heads cause I'm certain that the difference in the two is the height of the platform the springs sit on. On a standard head the springs sit on a platform that is machined lower (deeper) so longer valves in the standard head would mean the springs would have less preload which could be an issue performance wise cause the valves may not spring shut as fast as they would with normal spring tension. There would be less pressure on the rockers against the cam as well so you might get some slop/chatter at higher RPM's. |
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Post by Clank on Dec 3, 2014 1:29:37 GMT -5
Only specs I've ever found is either 69mm 64mm valves. And that's all I've found.
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Post by Clank on Dec 3, 2014 1:34:29 GMT -5
This is the bbk 64mm head with the 69mm valves installed  |
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Post by Clank on Dec 3, 2014 1:37:14 GMT -5
After swapping valves before install. 69mm valves in 64mm head.  |
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Post by Clank on Dec 3, 2014 1:39:19 GMT -5
But I put longer valves in a shorter valve head, so that would give them more preload, correct?
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