Streetable Big block

What is the biggest size pistons I should go with in a BB that is being used for a daily rider. I have heard not bigger than 1395 on a z1 900 with a J head.

I believe you are correct. Without an oil cooler, you will probly have overheating issues in stop and go traffic.

Some info:
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.

Good luck.

IMO i would not use a big block as a daily driver on account of heating issues associated with the big block. 2nd a Z1/KZ style head cannot flow what a big cc needs W/O major porting. i would think you would be better off with 1200 / 1260cc in a stock block with larger sleeves installed also the cylinder head would not run out of steam with the big cc i would like to hear Larry's thoughts on this.

Anything that big has potential for heat problems on the street in hot summer weather....but guys do them anyway.

At least you have a J head to start with. Not that it's a flow monster but it takes less work to port it for a big cc motor.

i think it depends on your def of a daily driver.
mine is a once a week driver, and the only street bike i own.
i have had a KZ1327cc big block. it ran 250*f . i could run it all day at speed but i wouldn’t want to get into stop and go traffic.
same with my stock block kz1200 but i have more lea way with the 1200, before heat issues, since it normally runs at a lower temp.
the BB don’t have air spacing between the cyl."s.
the J head would be nice on a 1200 also.


i have a freind with a 1428, but he doesnt ride in the city.

1395 is a good size as it leaves decent wall thickness. 1327 has wall thickness like a stock KZ