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Post by frankenscootdk on Apr 24, 2023 14:37:10 GMT -5
Does anyone know of a make/model that uses the long valve head for the qmb?
Would like to use one on my next project and im looking for a better quality part.
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Post by aeroxbud on Apr 24, 2023 14:42:14 GMT -5
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Post by frankenscootdk on Apr 24, 2023 14:48:20 GMT -5
Yes thank you, but still generic right?
My thought was maybe I could find a OE from a reputable or taiwanese brand
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Post by aeroxbud on Apr 24, 2023 14:51:26 GMT -5
Yes they are for the QMB139. They are very helpful if you contact them regarding what you want.
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Post by frankenscootdk on Apr 24, 2023 15:02:11 GMT -5
They are great and have reasonable shipping cost to my part of the world. But for this build i would like a branded top end if possible..
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Post by GrumpyUnk on Apr 24, 2023 15:35:15 GMT -5
you could try gy6store dot com.
Is the long valve head desired because that is what you currently have? The length of the valve stem does not
do anything for performance or any other effect to my knowledge. It may increase the moving mass a bit and thus make higher revs require a stronger valve spring to attain the same revs as a shorter, less mass, valve. Maybe.
I have not weighed them to know for sure.
If you want a Sym or Taiwan product, why not go to a Sym dealer? There is a company in Canada that GTvert has used for more 'name brand' products, though the name escapes...
tom
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Post by frankenscootdk on Apr 24, 2023 16:44:15 GMT -5
Currently i have a 64.
I also thought about that. The longer valve itself is not better.
My idea was that it could have an advantage in velocity if it has longer ports? Besides more material.
Springs i would match.
Good idea.
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Post by aeroxbud on Apr 25, 2023 4:53:00 GMT -5
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Post by frankenscootdk on Apr 26, 2023 4:46:57 GMT -5
Thank you much for your inputs guys.
The naraku is also the only branded part i can find that could be of better quality. But for its 2x price over stock maybe there was something else..
I found a sym also but it is at least 4x over stock.
If it has to be the 69 i dont know if i can justify anymore if it doesnt have better ports.
I also believe the chinese heads are all about the same and not that they are bad, but for the known branded ones i imagine the material quality and specs tolerance to be better?
It just seam out of place to just use the same cheap head after upgrading everything in between😅
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Post by GrumpyUnk on Apr 26, 2023 9:24:28 GMT -5
The 139QMB and GY6 are 'approved designs' in the Far East. They are allowed to be manufactured. I dunno if there are other designs, but from what I can tell, there is marginal difference. To be labeled as such, they have to use the same molds, machining, etc, to get 'universal fit'. Whether the alloy used is better/worse from brand to brand, I have no clue. It could be there are only a couple-three casting foundries and all the makers buy rough castings from them, and do finish machine work as appropriate.
If you want more performance, you might look into cleaning up the ports. I have not really looked, but bet there are some quick fixes that would make a bit of difference. Most likely the ports have ridges/bulges immediately under the valve seats that could be relieved a bit to allow cleaner flow. The seat diameter being larger than that of the port immediately into the port. You can watch some YT and see what is taken off. Maybe even what the effect is. I do not think you will find a LOT of difference.
tom
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Post by frankenscootdk on May 5, 2023 6:49:01 GMT -5
Thank you Tom,
Youre right. I believe there could be a difference in alloy quality, also some castings look worse than others. The measures are the same fit.
I got a naraku and it stands out from the generic chinese. Also in the feel of the material. Nice troughly machine work and didnt see flaws/ridges looking down the ports, no difference in size though. No champfer in the intake. No scratches at the gasket faces. Still have to take a look at the bowl area.
The branded chinese also looked better worked than the generic. And probably a sym would be nicely worked also.
If I were not to touch the head the naraku one is the best swap.
The branded seams like the best option because there is only minor work to do to it.
With the cheap one i can scrape material of with my fingernail so it has to be smoothed out.
I have not been able to find anything on yt about effects on this head. And only above 80cc the heads start to change..
Do you think it is worth it if also working on the chamber? There is a big increase in compression?
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Post by GrumpyUnk on May 5, 2023 10:03:40 GMT -5
I have not done any but 63 & 72cc changes. The 80 to me seems as if the CR would be pretty high.
Maybe there is something on site from someone who has done an 80cc. The difference I have seen in the heads designed for the larger bore are limited to some work on the surface of the head, cutting a shallow but wide groove to open the gap between bore and head out to the larger diameter. The others, not opened, will have a very thing squish area at the outer diameter of the piston where the combustion chamber gets extended out to the 'fire ring' or cylinder head gasket. I have not measured to see if the combustion chamber itself is larger.
tom
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Post by frankenscoot on May 8, 2023 6:41:38 GMT -5
Sorry, the 72cc is also the one i am currently at.
A goofit head on ebay has some different cutouts around the valves, don't know if you have seen it. But it is kind of that idea.
It is advised to keep the ø39 head, but at the samme time there could be compression issues.
Therefore i imagine if it is less optimal with the ø47 that maybe there is something to gain from the ø39 if making it more efficient.
Do you have an opinion on if the spark plug should be one number colder? I remember something about a 72cc to 80cc should have a cr8hsa at least in mopeds..
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