How do you put timing chain back on and time cams after head gasket change

First, DO NOT start the engine if the mechanical timing is wrong! My next obvious suggestion is to get it to a good mechanic and get them to sort it out for you.
I'm assuming your refering to the Mechanical Timing? Mazda use an 'Interference' engine so read this all the way through BEFORE doing any work. An interference engine means the valves will hit the piston face if the mechanical timing is even slightly out. That is why you don't start your engine with the timing set wrong. It's a VERY expensive repair.
Start from scratch. Remove the chain from the CAMs and cable tie it to something on the top of the engine to stop it disappearing out of sight.
Turn the Crank to about 10 to 15 degrees past Top Dead Centre (TDC). This is to move the pistons away from the valves so you can set the Cam timing marks. Set the Cam marks first then turn the Crank BACK to TDC. As long as the chain marker on the crank sprocket is lined up correctly the Cam chain markers should be correct.
Once you have timed everything into place use a socket and ratchet to turn the Crank pulley bolt Clockwise 2 full rotations. The timing marks should return to the correct positions.

Have it towed to the nearest ASE certified repair shop ! What's missing Tammy ? Is there spark ,Is there fuel pressure ? Did you have to take timing chain off to replace head gaskets ? are the timing marks set right ? Why did you replace cam sensor ? You need to find out what is missing first.

Yes they do. You'll probably have to hit the head with a rubber mallet a few times to loosen it up, but then it will just slips off and the pistons stay connected to the rods. You really need a shop manual for this because you'll have to take the cam chains off and then re-set the cam timing when you put it all back together. No big deal if you follow the instructions closely, but if you get it wrong you'll wreck the motor when you go to start it. Also, as long as you've got the motor apart you might consider changing the cam chains depending on how many miles the bike has on it.

there are timing marks on cam to line up with other cogs

Get Ford timing tool kit, part = otc-6488 (For about $119 USD on eBay)

4 timing chains (3 front including the optional balance shaft chain + 1 back)

2 cam guides (1 front + 1 back)

2 oil tensioners (1 front + 1 back)

1 jackshaft tensioner (1 front)

1 jackshaft guide/cassette (1 front)

1 balance shaft tensioner (1 front) (will probably be ok, so dont replace it)

1 balance shaft guide (1 front) (will probably be ok, so dont replace it)

new sprockets + bolts + gaskets etc.

new spark plugs + throttle body cleaner + lithium grease spray + oil change

head gaskets

some of these parts (the front stuff) can be obtained as a primary timing chain rattle noise kit Part# = 2u3e-6d256-** ab for 4*4 & bb for 2*4.

The kits also have later part numbers for us$ 76 @ http://www.fordpartsonline.com

My parts order (without the balance shaft stuff) was:

$76.04 KIT-TENSIONER TIMING 2u3e-6d256-ab

$47.93 Engine, Camshaft and timing, Timing chain, TIMING CHAIN, Explorer, Mountaineer, Rear - 4.0L SOHC - 4.0L SOHC

$57.36 Engine, Camshaft and timing, Chain guide, CHAIN GUIDE, Explorer, Mountaineer, Jackshaft To Cam - 4.0L SOHC - 4.0L SOHC

$103.22 Engine, Overhaul gasket set, OVERHAUL GASKET SET, Explorer, Mountaineer, Upper (Valve Grind) - 4.0L SOHC - 4.0L SOHC

$28.93 Engine, Camshaft and timing, Tensioner, TENSIONER, Explorer, Mountaineer, Upper - 4.0L SOHC - 4.0L SOHC

Its worth mentioning that this procedure is not enjoyable at all and should be undertaken with at least 8 full days to completion (I recon you could do it in half the time the second time).

It is basically on entire engine re-build with both heads off and the engine out of the vehicle.

It is not possible to do the rear chain or sprocket with the engine in the car as the flywheel needs to come off + at least 1 head, but it is possible to do the front primary chain and or front cam chain with the engine in the car.

You will be a ford exploder and timing expert after you do this - the main reason should be the love of your American built vehicle. Its a solid car - except for the plastic guides!

Disclaimer Note: This is how i successfully did the job, but i am not a ford mechanic and don't hold me responsible if this does not work for you.
Timing Overview

A Small Sprocket on the crank turns a larger (2 to 1 ratio) jackshaft sprocket that is in turn connected front & back to the 2 camshafts.

Thus a 360o turn of the crank will turn both the camshafts 180o.

Firing order is 1-4 2-5 3-6, Right side numbers 1,2,3. Left Side = 4,5,6.

At TDC pistons 1 & 5 are fully raised, thus 1 is about to spark & 5 is evacuated.

The engine will turn freely (no piston to valve contact) with the cams 180o out, it will even run (roughly) with 1 out 180o, any other settings may cause major damage to the valves.

The 2 cam shafts come with timing markings, "yes" real timing marks that anyone can use (even without special tools) to ensure correct timing.

The way to time it is to ensure both the off centre cam shaft slits are level/flat with the head.

With the engine at TDC both the cams need to have the off centre slot in the same position either up and level or down and level.

It is super critical that both the camshafts are 100% in sink with each other, i.e. not a few degrees off, the engine computer can compensate for crank timing but not engineering failure.

Be warned the camshafts can turn easily fast & hard by themselves as a few springs are compressed at TDC, I got my finger jammed and its still healing + if the engine is not at TDC you may damage a valve.

Now heres the hard thing that you will need special tools for - if you need to change the timing, you will need to be able to undo the cam sprocket bolt on both cams & these are done up real tight.

There is a tool that you attach that has 2 pencil thick shafts that fit into the sprocket and prevent it from turning - then you need to put about 90Nm of force on the bolt (remember the rear one if LHT)

For the $money$, its well worth getting yourself the tool kit - it will save you hours in the long run. Search eBay for otc-6488 should be US $119 - then sell them again for say $100.

There is also a tool version of the oil tensioner in the kit that screws right in and puts the correct amount of force on the chain + guide, to enable you to do up the cam bolt with the chain and sprocket in the right spot on the cam.

yes

please read all before starting not to bad to change did you check compression if your sure that's what it is its about 4hr job.start by putting #1 tdc intake side.note where vacuum hoses are can mark with gray or yellow perm marker remove breather all plug wires exhaust manifold then intake manifold remember what bolts in and whats nutted on.remove break booster cross vacuum hose and egr control valve on left rear of intake side remove vale cover ((you'll have to get something to hold tension on timing chain so not to let tentioner come out inside timing chain cover when removing came gear)) remove head bolts only in off order so not to put tension in one area. do not remove cam shaft bolts and do not turn cam shaft at all after timing gear is removed take off head and check piston walls for wear replace gasket and put back in GOOD LUCK

make sure #1 clyinder at TDC(top dead center)
line up timing marks

To replace the head gasket disconnect the battery, remove the tank, remove the rocker cover. Manually turn the crank shaft until the engine is on top dead center compression stroke and the lines on the cam shaft gears are even with the top of the cylinder head. At that point evenly loosen the bolts that hold the cam shaft. Do one cam shaft at a time. Before you remove the second cam shaft tie a piece of wire to the cam chain then let the chain fall into the hole after removing the second cam. Loosen the carburator mounts and slide the carburators back so they are free of the engine. Now you can evenly loosen the head bolts and then remove the head. Check the head and cylinder for signs of warping or why the gasket needs to be changed. When you replace the head gasket make sure the gasket fits over the guide pins. Reassembly is essentially reverse of the disassembly. After you get the head torqued to specs, you will be glad you fought that wire attached to the cam chain when you start to install the cams. Make sure each cam goes back from where you took it from and the lines line up like they were before you took them out. Don't forget to install a new gasket on the rocker box.

no adjustment neccesary. There is a timing chain tensioner, which you will simply push back in to its body. once you do that, there should be a small hple somewhere on the body of the tensioner. That' sthere so you can piut a paper clip into it to keep the tensioner from coming out. Once you lock that in place you can remove the tensioner and than the timing chain.Once you get it all back together, just pull back on the pin and the tensioner will come out and put tension on the chain. good luck.