Head differences

My 76 kz900 started blowing oil out of the right side pipes. I was figuring I would need rings. I did a compression test (hot) and got 80, 108, 0, and 80 pulled the head off and found the #3 exh valve with a hole in it. found the oil problem. will the head off of a 1000 work on it. it would be a quicker fix to get the bike back on the road or can i change just the bad valve. I know I should just save my money and get the head rebuilt and maybe pistons and rings. Thanks for the help/advice.

if the 1000 head is in good shape then bolt it on they are almost exactly the same, very small differences then get the 900 head rebuilt in your own time

Valve sizes are slightly different and the exhaust studs are 8mm instead of 6mm so you may want to see if the exhaust collars you are running will fit. Other than that ,it will bolt right up.

the 1000 head I can get is later year 1982 mine is 76 would that be a problem? thanks

Differences between KZ & J Motors from APE's Website


There are two different KZ1000 engines. The first is the Z1 type which was built between 1976 and 1980. The second is the 1000J, or "J model" as it is called, and was built from 1981 until present ( currently only available in police motorcycles). These engines are different, but some of the parts can be interchanged, and this has caused much confusion. We hope this makes everything clearer.

The first example is simply a big bore Z1 900. This engine came with a kick starter and electric starter. The cam chain tensioner hole in the cylinder block is a rectangle. The ccs cast on the cylinder is 1015. The later "J model" has no kick starter, and the cam chain tensioner hole in the cylinder block is round. The cc's cast on the cylinder is 998. This engine also became the GPZ1100 in 1981 and '82, and virtually everything between them is the same. (The '83-'84 GPZ1100 was changed). Cylinder blocks and lower engine cases are NOT interchangeable. You can not put a J model cylinder on an older style lower end, and vise versa.

Cylinder blocks.. Not interchangeable. If you have a J model block/GPZ1100 block you must use a J model or GPZ1100 cases.

Crankshafts... Early KZ1000 used a roller type cam chain and the crank has that style center sprocket. J model used Hy-vo style chain. J model cranks can be used in early lower end if the center crank pin is changed to one with a roller chain sprocket. The J model connecting rods are approx 1.2 mm shorter than early style rods.

Cylinder head.. J model head has larger valves and ports. This head can be used on earlier style cylinder and lower end by installing APE #CCIA cam chain sprocket adapter in head . How to tell which head you have.. Early style heads have four factory drilled and tapped 6mm holes in the center to bolt in cam chain idler gear. Z1 900 heads have 6 mm exhaust studs, KZ1000 8 mm. On the J model head, the four cam plugs at the ends of the camshaft areas are bolted in, on the early style head, they just push in and the valve cover holds them in. The J model / '81-'82 GPZ1100 cylinder head has a larger combustion chamber than the early style head. When these heads are used on the early style motor, a compression ratio drop of about 2 full points can be expected.

Pistons... All pistons have 17 mm pin holes, but the J model/ '81-'82 GPZ1100 pistons have the pin hole lower in the piston to make up for the short connecting rod. All J model and early '81-'82 GPZ1100 pistons are the same ( '83-'84 GPZ has 18 mm pins ) Therefore any '81-'82 GPZ piston can be fitted to a J model engine to make a big bore engine. Pistons for the J model / '81-'82 GPZ1100 can not be used in a motor that has an early style long connecting rods unless a spacer plate is used under the cylinder block to compensate. If early style KZ pistons are used in an engine that has the short J model / '81-'82 GPZ1100 connecting rods, a special short cylinder block has to be used.

Enjoy...

All early kickstarter equipped 900 and 1000 zeds have the same size valves. Just bigger 8mm exhaust studs on the later models.
Otherwise a straight swap.
However to fit a later 1980 on (no kickstarter) - J/GPZ style head (Which does have bigger valves) You need an APE idler conversion which requires a small amount of machining.
These heads have a bigger combustion chamber (70mm sphere compared to the earlier 66mm)so you will lose 2 points of compression ( Knocks your 8.7 down to 6.7) but you can get most of it back by skimming.(you can safely take 1.3mm off the gasket face).

Sounds like it'll be cheaper to fix the valve if you just want a casual runner but the J head flows more in std form than a ported early head so it's a good basis for a fast motor if you were that way inclined.