GPZ bathtub head on early engine

[speed111]

I'm guessing this has been done many times as you can buy the APE conversion tower. Before I start drilling holes am I on the right track?
I'm thinking Uni head and cams 1075 kit and RS34's to make a decent road going Zed
Issues I can think of are compression ratio and valve clearance and carb settings
What do you have to machine to make it work?
And what are the results??

[zed1015]

The GPZ1100A untrack head has a steeper exhaust valve angle and you will need pistons with a modified exhaust pocket to suit that and to clear the bathtubs combustion chambers increased squish area.
Normal hi comp 1075 pistons for an early Z will hit the head
For a straight forward swap you should use a 1000J/GPZ B2 head which has the same valve angles as the early Z and a full hemi combustion chamber to clear the piston crowns.
Take note that the 4cc larger combustion chamber volume of the later heads ( from 36cc to 40cc ) drops the compression down approx two points so 10 to 1 will become 8 to 1 etc so either go for 12 to 1 pistons to get 10 to 1 or skim the head approx 1.3mm to to regain compression.
The J/GPZ valves are sunk into the head a further 1mm with 1mm shorter stems so going the skim route doesn't cause valve to piston clearance issues.

[ADRIAN H]

Use a J head to avoid valve / piston clash - As zed1015 (Rob) says.

You’ll need a top cam chain idler adapter and the jig for drilling. Second item down on the link.
https://kzzone.com/sprockets.html

[speed111]

I understand zed1015 but I've got a good GPZ head and finding a good J has not been that easy
Does sound like a bit of machine work but a head skim and exhaust valve pocket rework on 12:1 pistons not so bad and obviously the idle wheel conversion
What is the power like after the work? do you gain top end power at the expense of mid range grunt?
Or is a J head or flowed std head a better bet

[zed1015]

If you try to make the UNI head work with 12.1 pistons you will be up to your neck in piston mods that won't be worth the end result if you do have any crown thickness left.
This is why i recently sold my own uni head and sourced a J instead for this exact same type of engine build using Wiseco 12.1 pistons.
The stock un cut UNI head will just work on a J motor with stock comp J pistons and re-cut exhaust pocket to give a usefull performance increase for the J but once you get to using hi comp pistons you need modified crowns to match the uni combustion chamber.
Personally i would see if someone wants to trade a good J head for the UNI.
The simplest way is usually the best and for a 1075 the stock Z head with a proper port job and bigger valves will be plenty good enough.( I have three quick 1075 zeds with stock ported heads) .
Following that the J is the next step up but the increased flow rate of one ported is not overly beneficial until the capacity is in the 1200cc and above region where a fully ported Z head begins to become a restriction.

[Z1streetfighter71]

What kind of hurry are you in? I mean i have a J engine complete. Ill be rebuilding a GPZ 1100 this summer, we can do a swap if its the path you want to go down but im heading away for a month to Cambodia/Vietnam but ill be back mid march. The engine is out of a 82 KZ1000 LTD with 32k.

[speed111]

If you try to make the UNI head work with 12.1 pistons you will be up to your neck in piston mods that won't be worth the end result if you do have any crown thickness left.
This is why i recently sold my own uni head and sourced a J instead for this exact same type of engine build using Wiseco 12.1 pistons.
The stock un cut UNI head will just work on a J motor with stock comp J pistons and re-cut exhaust pocket to give a usefull performance increase for the J but once you get to using hi comp pistons you need modified crowns to match the uni combustion chamber.
Personally i would see if someone wants to trade a good J head for the UNI.
The simplest way is usually the best and for a 1075 the stock Z head with a proper port job and bigger valves will be plenty good enough.( I have three quick 1075 zeds with stock ported heads) .
Following that the J is the next step up but the increased flow rate of one ported is not overly beneficial until the capacity is in the 1200cc and above region where a fully ported Z head begins to become a restriction.

I'll get the std head flowed!
thought it might have been an easier cheaper route thanks for the heads up
Uni cams are a good move though??

[speed111]

What kind of hurry are you in? I mean i have a J engine complete. Ill be rebuilding a GPZ 1100 this summer, we can do a swap if its the path you want to go down but im heading away for a month to Cambodia/Vietnam but ill be back mid march. The engine is out of a 82 KZ1000 LTD with 32k.

I will keep that in mind
no rush been building this AC replica for ages

[Z1streetfighter71]

All good, wont be touching the J motor anytime soon so if you need it its there, ill be dropping in here from time to time but it will be there when i get back.

[zed1015]

Uni cams are a good move though??

Yes! Uni cams are the sportiest of the factory cams so the best option without buying aftermarket performance cams.

[thebuelligan]

The GPZ1100A untrack head has a steeper exhaust valve angle and you will need pistons with a modified exhaust pocket to suit that and to clear the bathtubs combustion chambers increased squish area.
Normal hi comp 1075 pistons for an early Z will hit the head
For a straight forward swap you should use a 1000J/GPZ B2 head which has the same valve angles as the early Z and a full hemi combustion chamber to clear the piston crowns.
Take note that the 4cc larger combustion chamber volume of the later heads ( from 36cc to 40cc ) drops the compression down approx two points so 10 to 1 will become 8 to 1 etc so either go for 12 to 1 pistons to get 10 to 1 or skim the head approx 1.3mm to to regain compression.
The J/GPZ valves are sunk into the head a further 1mm with 1mm shorter stems so going the skim route doesn't cause valve to piston clearance issues.

Would you see the same issue with the unitrac head and MTC 10.5:1 pistons Rob? Running a turbo motor so wanting keep the 2 point comp drop.Thinking I will still need to mod the valve pockets to be on the safe side

[zed1015]

Would you see the same issue with the unitrac head and MTC 10.5:1 pistons Rob? Running a turbo motor so wanting keep the 2 point comp drop.Thinking I will still need to mod the valve pockets to be on the safe side

Any Z piston ( except a flat 8-1 turbo piston ) needs the EXHAUST pocket angle on the piston modifying to match the uni valve and the higher the dome you run into clearance issues with the UNI's squish band which may mean special pistons or spacing the barrel and losing even more comp.
I'm doing the same with my turbo motor with Wiseco 10.25 pistons but using a J head after again deciding the work for the UNI wasn't worth it and for a turbo motor i don't think it would really make a noticable difference over the J especially once i've fitted bigger valves, opened the throats and ported the exhaust side.

[thebuelligan]

Would you see the same issue with the unitrac head and MTC 10.5:1 pistons Rob? Running a turbo motor so wanting keep the 2 point comp drop.Thinking I will still need to mod the valve pockets to be on the safe side

Any Z piston ( except a flat 8-1 turbo piston ) needs the EXHAUST pocket angle on the piston modifying to match the uni valve.

Thought that would be the case Thanks

[Al]

The area highlighted in blue is the area of the pocket that needs to be cut for the different angle on the Unitrak exhaust valves.
The slender elipse at the bottom of the pocket was the original cut from Wiseco.
These are 10.5:1 1170 Wiseco 17mm J pistons designed to be used with the J head's combustion chamber diameter. Hence the massive land round the circumference.
AL

[thebuelligan]

The area highlighted in blue is the area of the pocket that needs to be cut for the different angle on the Unitrak exhaust valves.
The slender elipse at the bottom of the pocket was the original cut from Wiseco.
These are 10.5:1 1170 Wiseco 17mm J pistons designed to be used with the J head's combustion chamber diameter. Hence the massive land round the circumference.
AL
Wiseco cut exhaust pocket.jpg

Thanks Very helpful