Cylinder Head Replacement

[GilHirien]

Hello all, been a while since I have posted anything, but I still ghost around the forums now and again. I recently had the valve seat for my exhaust valve decide it did not want to exist anymore so it crumbled and threw itself into my piston only to be hammered back to the head. It manifested rather quickly over the course of a few days with no sounds to warn me. It just seemed to be harder to start, and then only want to run with a little throttle applied. This went from almost not noticeable for the first two days to a major pain on the third. The final start resulting in a horrible sound of catastrophe and it was over. I pulled the head to survey the damage and I was prepared for the carnage but my heart still sank a bit. Pics will follow when I get the time.

I was then in need of a quick replacement, as cheap as I could find and have reasonable expectation of working. This is my daily driver and my commute is an hour one way. Noting that the cylinder and head have remained nearly unchanged since the gn250 in '82 I searched away for a decent used cylinder head that I could mount on a shoestring budget. The following is for information and posterity for those who face similar troubles and I do not warrant or guarantee its success if you jump to it without knowing what you are doing.

I was able to source an '85 LT250EF (Suzuki ATV) cylinder head which mounts nicely to the stock cylinder. Note that the crankcase is quite different between the two, but the head is very nearly unchanged in 20 years. A few things of note, most of which I expected going in to the project: The intake boot of the LT250 features a vacuum port for the petcock instead of the one located on the GZ250 carburetor. You can use either as there is no difference in function or size of hose, but the other must be capped/plugged. Additionally, the LT exhaust bolts are M8 size rather than the GZ M6. You must either find a reducing bolt (available, but hard to source) or be prepared to drill out the exhaust mounting plates for the larger bolts (I did this. There is enough meat on the plates, but keep it as small as you can make fit over the bolts). I knew of these before hand from hours of research.

Here are a few other things to consider: The casting for the head is slightly (very slightly) different with a few minor exceptions. there is more empty space behind the sparkplug hole on the LT than the GZ and the fining is a little different. There is a little more fin on the older LT and has worked quite well thus far. The engine does not seem to run any hotter or significantly cooler.

Here is the real crux and the thing you should be most aware of if you decide to pursue this replacement option: The cam shaft mounting in the cylinder head. The pocket for the shaft at the lobes and rear are identical and will provide no problems. The shaft forward of the timing gear however is significantly larger diameter on the GZ than the LT. The GZ accommodates this by including a larger cutaway in the casting. Note that it does not contact the head on this side. The LT head does not have such a large cutaway and was simply machined the same diameter as the camshaft minus the lobes. You will have to cut away some material for the camshaft to rest in its groove without contacting the head past the timing gear. I recommend you machine it if you have the capabilities (I am a machinist) but it is not necessary as this surface does not contact any part nor support the camshaft. I simply drilled about an eighth inch outward of the existing groove with an eighth inch bit moving my way around the cutout to remove material. I then filed away at the remaining material making sure I had clearance all the way around. I do recommend machining this out though for a better finish but functionally it makes no difference.

I have since put around 200 miles or so on the swapped head with no ill effects. Gaskets are the same for the cylinder and head on both GZ and LT. Make sure to check valve clearances and adjust if necessary. Ride on!