Yamaha NXC125 20,000km

[speedpro]

Couple more photos. the crank and head are SD branded and the piston is FJN. Everything seems pretty good quality and nicely finished apart from the previously mentioned porting and piston surface.

[speedpro]

Is there only two squish areas rather than 4 on that piston head combo?

More or less. There is just a small perimeter each side with matching flat surfaces. With larger valve sizes for the given bore having a squish area between exhaust and intake each side would mask the valves I would think. The only time it wouldn't be masking the valves is if the valves were at a very shallow angle and the valve seats were more or less in line with the flat plane of the head. Then as soon as the valves lifted they would be in free space with no combustion chamber surface close to their perimeter. This sort of thing: https://www.psychobike.com/forums/10...n-chamber.html.

The downside might be a lack of swirl in the chamber which back in the old days was a big deal. I'm thinking of Suzuki's TSCC system. I think in that case that the majority of flow was from one side of the valves which probably happens in any case with port directions and valve masking though good port design should reduce it a lot.

[husaberg]

More or less. There is just a small perimeter each side with matching flat surfaces. With larger valve sizes for the given bore having a squish area between exhaust and intake each side would mask the valves I would think

I was just basing it on say a CRF150R
which would be similar bore.
22.5 and 26mm valves
I think even the old VT250 were similar to the CRF150R



What size is the pin on the piston 15mm or 16mm

[speedpro]

Those valve seats are a lot further out and closer to the plane of the head gasket surface compared to the scooter head, by the looks. I would think it would flow better but would need to see it on the cylinder to see if the side of the bore would have an effect. Flow bench wouldn't lie of course. Being a very flat valve angle would also effect the piston dome profile and as you say the CRF has the flats between the exhaust and inlet valves and there's a bit matching it on the head. The scooter head is probably a bit older style.

[speedpro]

Some real nerve wracking grinding this weekend. I had to grind down real close to the valve seats without touching where the valves seal. It wasn't too bad cleaning out the ports and I only had to take out a small amount to clean up where the valve seats and the actual port meet. Comparing it to the old Taida head I actually think the Taida head has slightly a better intake port. The exhaust port is about the same. I've got it sitting on the vice at the moment with the combustion chamber full of WD40. There may be just a very small amount got through on both sides but it may also be oil from fitting the valves in the head. I'll know tomorrow morning. The final photo looks a lot worse than it really is but I would still prefer it to be better.

[speedpro]

Coming along. Getting the roller rockers in were a mission as usual. It's a tight fit inside the head and the rockers have to be slipped in more or less to where they go and then pushed up out of the way to fit the cam. Once the cam is in the rockers can be slid down to where they go and the pivot pins put in. Everything else just bolted together like normal with the spacer plate and extra gaskets.