It's been well documented on here on how BIS engines love to eat a head gasket or two, in fact if you're a "Boils In Seconds" owner and you've not suffered the woes of a steamy engine on the side of the road yet you may be feeling left out like your missing out on a right of passage. Luckily doing a head gasket change is relitivly simple for the average home spanner flinger and can be done without removing the engine in a mornings work.
So first up you need to drain the engine oil and the coolant into a suitable container so ideally you need the back end in the air around 8 inches at least. Start with the coolant first by undoing the big hose going to the thermostat and pulling it off gently. This will drain the coolant from the radiator. Next undo (but do not remove) the two 13mm bolts holding the thermostat on. Wiggle it to break the seal. This will drain the coolant from the engine. To get as much out as possible remove the filler cap and the little bleed screw on top of the heater matrix which is at the front of the car so atmospheric pressure can let more coolant out. With as much out as possible refit the hose and tighten up the to thermostat bolts. Next up undo the sump plug and drain the engine oil. Whilst that dripping away remove the carb top (not the filter box) spark plugs, the HT Leads, and all the vacuum hoses from the carb. Also extract the choke cable and the disconnect the throttle cable from the carb linkage an tie them out the way
Undo and disconnect the three hoses from the coolant junction that's on the cylinder head, word of warning this junction was cast using a mixture of biscuits and paper so is very brittle! So be careful when removing the hoses - try not to lever. Also pull the connector off the temp sensor. Now the exhaust. There's bolts and nuts dotted all over the place. Two 13mm nuts on each of the two flanges, two 13mm bolts on both of the brackets and one 13mm nut hidden at the bottom of the head between the silencer part of the exhaust. Once all of these are out its a case of wiggling and threading the whole system out over the rocker cover and out the bottom of the car. Once out remove the two 13mm bolts holding the rocker cover on and take it off. It may be a little tight so a light tap with a hammer will see it come loose.
Back at the top it's time to remove the carb. The easiest way to do this is to use a cut down 10mm spanner as space is tight. It's a bit fidley but eventually they do come off. Remove the fuel feed pipe from the pump to carb and then lift off both the carb and the base plate being careful not to tilt the carb as its full of fuel. Once off undo the three 13mm bolts that hold the manifold on and slide it off.
Down below again and the rocker shaft can now come off. There's two 13mm nuts holding the shaft on (shown with a spanner on in photo) so undo these and wiggle the rocker shaft assembly off. To give you more space when sliding the head off you also need to remove the studs using either a stud extractor or two nuts locked together. Gently pull the push rods out as well one by one making a note of which hole they came from.
Now it’s time to remove the domed head nuts. There’s 8 of them with the top two inner ones hidden in the inlet ports. Use a breaker bar and a 17mm socket for these starting on the inner ones first then the four outers. There's also two 13mm bots right at the bottom and also a stud/bolt right at the bottom corner which the exhaust bolted too.
Depending on the condition of your engine the head may simply slide off. If it’s tight then use a rubber mallet to tap on different sides of the head until it breakers free - don’t smash the hell out of it with a ball pien hammer as you’ll just damage the casing.
The cooling system on the BIS isn’t a bad system really it just needs to be in tip top condition. Don’t forget Fiat didn’t knock it up in a week. There would’ve been 1000s of miles of testing so essentially it’s a sound set up however there can be no blockages or air locks and no leaks which let air back into the system. Also the two fan speeds need to work properly - this is crucial as the faster stage two speed cuts in at 103 degrees which is rather too late.
You can see here on the bottom left that the gasket has failed and gasses have passed though. This is the cylinder nearest the back of the car.
The refitting process is pretty much the reversal of the removal however there are some things to consider.
Preparing the head - You must clean off all of the old gasket as possible. Use a scrapper to get stubborn gasket off then use different grades of wet and dry on a block to get the surface really clean and smooth. If your engine has suffered a catastrophic gasket blow you may need to get the head skimmed however in my experience in most cases you just need to scrape then a good clean up with wet and dry will be suffice.
Valve clearance - When you’ve refitted the rocker shaft you’ll need to readjust the valve clearances. A fiddly time consuming (but vital) process. You need to turn the engine over by hand until the piston is on its compression stroke and both inlet and exhaust valves are closed. Using a slip gauge (or feeler gauge) set to 0.15mm check the clearance by inserting the gauge between the valve and the rocker. If it’s out you adjust it by undoing the 10mm nut ant turning the adjustment screw until you get the right setting then lock the nut up. You kind of need three hands but there are special tools on the market to make to job easier.
Torquing the head - This needs to be done in a certain sequence. Working from the centre nuts first you tighten them in two stages - 25Nm first then again at 49Nm.
aftercare - Unfortunately it is necessary to retorque the cylinder head after 500 miles. This is something you cannot ignore as the gasket compresses over this period and if left you run the risk of eventual loss of compression and you’ll have to do the job over again. And even more annoying is that you practically have to strip it all down again as two of the nuts are hidden behind the inlet manifold and four others are tucked behind the rocker shaft. This means draining the coolant from the engine and of course another oil change.
Tools you’ll need10 and 13mm are the main sizes of fasteners so sockets and spanner’s will do plus 17mm socket for the head nuts. You’ll need a scrapper for cleaning up the head, screw drivers for the jubilee clips, a rubber mallet a torque wrench, feeler gauges, suitable containers to drain into, wet and dry paper and something to get the car up in the air so nothing too drastic
Cut down 10mm spanner for carb
Tool for setting the valve clearance
These engines and thier aircooled counterparts are a joy to work on. It’s mechanics at its most basic so take your time don’t not rush - you may even enjoy it!
big thanks to xylaquin for letting me use his car for this guide