assuming you have the 787 engine:
this sounds like the head pipe gasket to manifold which is common area for a leak.
Use OEM gaskets as they are thick and forgiving of a few irregular surface imperfections. Use NO RTV sealant on the OEM gasket between head pipe and manifold and follow strict torque specs for the 3 allen bolts and one stud/nut tightening torque.
if you can avoid it, do not loosen the large V band clamp that holds the Tuned pipe to the head pipe or you will need to reset the orientation of the pipe...
what you could do is:
loosen one clamp on the cooling hose from the head to head pipe and remove the cooling hose from one of the hose barbs, head or pipe will do.
remove the lower 2 allen socket head bolts and washers that hold the tuned pipe to the engine case, undo the hose clamp to the rubber coupler between the tuned pipe and water box connection, remove the bolt that holds the head pipe to bracket on the mag housing, remove the 3 allen head bolts and 1 nut/washer securing the head pipe to manifold, try and slide the whole pipe forward towards the fuel tank and off the studs enough to clean the old gasket off and install the new gasket...
try and use a new OEM gasket with NO RTV sealant. Copper spray is OK and what I use.
If you use a SBT or other metal shim gasket it is ok to use Permatex Copper spray a gasket on all the surfaces and both side of the gasket, let tack up and install...
I strongly suggest downloading a shop service manual and follow the torque specs and the procedure for tightening or pipe placement if you get into a jam... or a misaligned pipe will blow out the gasket again and or crack the pipe...
after the gasket is in place, slide the pipe back over the studs, hand tighten the allen socket bolts and stud/nut enough to pull the pipe in and start the bottom 2 allen bolts down alongside the cylinders. This is providing you didn`t loosen or remove the V band clamp and copper sealing ring, otherwise you will need to start fresh and reorient the pipes.
don`t forget to use loctite on all the bolts/nut, I`m pretty sure it is 242, Blue...
if you did separate the tuned pipe from the head pipe, check to see if the copper sealing ring was crushed or leaking, clean the surface very well, you may need a new copper sealing ring and this area is sealed with RED high temp RTV during assembly of this joint...
the easiest thing to do prior to pipe separation is make a mark across the joint so you know where it belongs prior to breaking the seal...
it is much more easier to show, than explain it in text...
continuing:
after the bottom bolts are started, install the head pipe to bracket bolt as well, tighten the head pipe to manifold allen bolts and stud/nut to spec, tighten the bottom 2 tuned pipe bolts to engine case, and tighten the head pipe to bracket.
this is assuming you did not separate the head pipe from the tuned pipe V band joint... if you di separate I would do a mock up dry (no gaskets to align the pipes and then do it again as if your installing the new head pipe gasket...
a shop service manual will def help...
good luck...
