Valve question

[slawenm]

Been reading through the great forums for a while now and I can mostly find answers to questions by searching but this time I have what to me seems to be a stupid question I think I already know the answer to.... however I shall ask anyway.

Forgot to give some info my bike is a Lexmoto 49cc 4 stroke using the GY6 139QMB engine

I bought a new 72cc BBK kit complete with a larger head I've not yet fitted the kit however today I noticed the head didnt come with any valves.
first question can you swop out valves from another head into the new head (I think the answer is no because I doubt they are interchangeable)

Second I can only find valves for sale for a 50cc head not the large 72cc 47mm larger head will the 50cc valves fit or even do the job.

I cannot use my old head because I split it using an impact driver to get some busted exhaust studs out.

Many thanks need an answer soon as I have a friend coming to install the kit for me this weekend YIKES!

[slawenm]

Oh and if it helps I measured my intake and exhaust size and they are 18mm intake 15mm exhaust sure that is the standard 4 stroke head sizes so I doubt this really is a "larger head" will the fact the intake and exhaust sizes are the same as a 49cc be a plus factor to getting away with throwing some standard valves on the head?

[Fox]

Just went out and measured a stock head. Intake 18mm, exhaust 15mm.

The head you have may be a larger diameter where it starts to dome up to work better with the bigger bore/piston. You'll be able to tell once you get them both side-by-side each other. I tried a 70cc piston and barrel with a standard head and got poor results. Good luck! Keep us posted! Start a thread in the builds section when you get it together.

[slawenm]

Thanks for a quick reply yeah I'll have to get posting now I popped my cherry so to speak, I will try swap out the valves first and if that fails buy new ones don't want to be spending more money than I need to and the old head is going to be binned anyway so if I do damage the valves trying to remove them ahh well

[Fox]

You hold the head upside down and compress the springs while you hold the valve from moving with your thumb and the keepers fall out. Then once you put the valves in the other head you put a bunched up rag on the bench and set the head on it. The rag holds them up and then you set the keepers in position and compress the springs down and let gravity drop them into place. I made a tool for doing them out of some pipe fittings. I'll post some pics tomorrow of the tool and how it works if you want.

Edit: Brent may already have a video in the library somewhere or a how-to with pics.

[slawenm]

Thanks for the info on removable yeah some pics would be great I have all sorts of sized copper piping laying around from plumbing some stuff in the bathroom, anything to make the job easier come weekend

Edit: I've looked through most of that stuff and couldnt find much on the how to I managed to find a post of Brent saying somebody could try swap out valves from a 50cc head to 72cc head

[Fox]

You may not have to swap 'em out cause that "big" head you have may be the same as the stock head. People will say all sorts of things to get you to buy stuff.

I just went out and measured the stock head. It's 39mm where it starts to dome up.

[slawenm]

Aye this head domes up at 39mm too, as I said it came with the kit and I need anew head cause my other is split.

as for swapping valves my new head has come fitted without any which is why I'm hoping to take them off my split head and put them in this new one however appears both heads are the same so swapping the valves out is in fact an option

The 72cc BBK was just a little treat really I needed to replace the head and thought while its apart why not grab some extra power just hope the compression with this head wont be to high.

[Fox]

Oh right! I forgot the old head was knackered.

[speedy1125]

if you use the old valves in the new head you need to lap them so the seats and the valves match as much as possible.

[speedy1125]

Nov 9, 2011 21:10:38 GMT -5 slawenm said:

Aye this head domes up at 39mm too, as I said it came with the kit and I need anew head cause my other is split.

as for swapping valves my new head has come fitted without any which is why I'm hoping to take them off my split head and put them in this new one however appears both heads are the same so swapping the valves out is in fact an option

The 72cc BBK was just a little treat really I needed to replace the head and thought while its apart why not grab some extra power just hope the compression with this head wont be to high.

if its a 39mm stock head then you will need to use 2 base gaskets or you will bust the piston. I used a 47mm head with a 50mm piston and it lasted about 350 miles.

[speedy1125]

Nov 9, 2011 21:10:38 GMT -5 slawenm said:

Aye this head domes up at 39mm too, as I said it came with the kit and I need anew head cause my other is split.

as for swapping valves my new head has come fitted without any which is why I'm hoping to take them off my split head and put them in this new one however appears both heads are the same so swapping the valves out is in fact an option

The 72cc BBK was just a little treat really I needed to replace the head and thought while its apart why not grab some extra power just hope the compression with this head wont be to high.

if its a 39mm stock head then you will need to use 2 base gaskets or you will bust the piston. I used a 47mm head with a 50mm piston and it lasted about 350 miles.
The guy i bought the parts from switched the head on me and kept the correct 50mm head and i did not realize it

[90GTVert]

I've never done a how-to for valve swaps. I use a PitPosse valve spring compressor. Seemed like it was worth the money for me, since I like to mess with engines whenever I get the chance. It's a very nice part for the price.

pitposse.com/vaspco.html


Here's a cheap one that would probably work, but I'd spend the extra on the one above if buying.

dratv.com/vaspco.html


Here's a link, but it's really simple if you have the tool.

scootdawg.proboards.com/index.cgi?board=125ccandlarger4strokegy6&action=display&thread=21458



Some info from an old post when I was messing around with the 4T I had...

May 22, 2010 13:34:57 GMT -5 90GTVert said:

IStill not sure how everyone else bolts on these 50 and 70cc heads with no issues. Maybe there is more freaky stuff about this SunL than I have realized?

I've been trying to figure out what the compression would be roughly. Not sure why.... I've never had much luck with compression numbers proving anything real world with scooters so far. lol Here goes anyway...

I cc'd the combustion chamber and got 5cc. That's what I expected since this stock head is beveled like the 47mm head. I measured the head gasket compressed thickness and bore as well and measured the distance the piston sits below deck. Still won't be completely accurate since I'm not accounting for valve reliefs or any variation in piston shape (it's pretty flat though).

Cylinder Volume : 81.3cc
Combustion Chamber Volume : 5cc
Head Gasket Volume : 0.5cc
Volume Between Piston Crown And Deck : 0.6cc
Total Volume Above Piston : 6.1cc

Compression Ratio : 14.3:1

For the sake of comparison, I ran a few other figures...

47mm bore w/ same deck/gasket thickness/head: 13.0:1
39mm bore w/ same deck/gasket thickness/head : 9.7:1
47mm bore w/ same deck/gasket thickness 50cc head : 14.1:1
39mm bore w/ same deck/gasket thickness 50cc head : 10.5:1

[Fox]

Seeing that tool makes me see how I can improve upon the tool I made.

As for lapping, I've changed valves from one head to another before and not lapped them and the engine ran fine.

[slawenm]

Well the install was today and all I managed to get done with the piston and new cylinder my attempt to swap over the valves was a failure spent hours getting them out and I cannot refit them on my new head because I cant press the springs in far enough with my hands and any tools or method I've tried to do so far has failed too.

not worth me buying the tool to put the springs in with because a new head with valves in would cost around the same... a friend is bringing over a G clamp tomorrow and we're see if I can fit them, quick question how many Dowels should I have all together cant help but think I may be missing some