To my other GT6 pages

March 21, 2020

Cylinder Head

The straight six engine in the GT6 is certainly outdated by modern standards, and was a pretty mature design even in its own day.  It's a very conventional overhead valve pushrod engine.  The cylinder head is a beefy 60-pound lump of cast iron that holds the combustion chambers and their two vertical valves each.

The first task on the cylinder head was to detach it from the engine.  I understand that this can sometimes be difficult, but in this case, the head came along peacefully.





I cleaned off a bench for disassembly and inspection.  There weren't any big surprises.




Disassembly was fairly routine.  The water jacket plug at the rear showed a little defiance.  I had to drill it out.




Also, the core plugs didn't respond to polite requests.




The remaining studs and valve gear came away without too much undue violence.




The only remaining removable items on the head were the valve guides.  The valve stems didn't feel too bad in the guides.  The valve stems did measure slightly out of spec on the small side, but this was compensated for by a "knurl" on the inside of the guides.  According to some notes I found in my Haynes manual, this is something I had done in the early 80s to address the stem wear.  Also, a couple of the guides were chipped on top.




All of this was enough for me to decide to replace the guides, so out they came.  The piece to the right of the guides is the drift I used to push them out.




So at this point, I had a bare head, and the usual thing would be to clean it up.  It is common to have the things like this "tanked", or "hot dipped".  These processes are usually a hot caustic solution that dissolves away the burnt, caked oil, grease and carbon.  I've had it done before, and was often disappointed.  It didn't seem to do much for the rust, or for the internal scale in the coolant jacket.  With judicious choice of chemicals, I thought I might be able to do as well or better at home.

I took the head to one of those DIY car washes with the high pressure hot soapy water to clean off what it could--mostly the oil and some of the carbon.  The rest of the carbon and the combustion chamber deposits came off pretty easily manually.




What then remained was rust and scale, and these things are better addressed with acids.  I made a box, lined it with plastic, and put the head in it.




Then added enough phosphoric acid to fill and cover the head.  Though there are stronger, quicker acids, phosphoric has the advantage of being a little safer to use, and it doesn't stink.




An hour later, the foaming suggested that the acid was doing something.  Phosphoric acid at this concentration has very little effect on metallic iron, but does attack other substances.  I left the head in the bath over night.




After a rinse and compressed air dry the next day, the head looked quite a bit cleaner, especially inside.




I had been thinking all through this process about possibly bumping up the compression ratio a little bit on this car.  I did some research to try to find out what the stock CR was, and was met with confusion.  The ratio varied not only by model and year, but also my market.  I read figures between 7.5 and 9.5.  The only way I saw to be sure was to calculate it myself.  One of the most important inputs for this calculation is the combustion chamber volume, and I'd have to measure it.

This is not frontier science.  People have been doing it for many decades.  It just involves measuring how much liquid it takes to fill the chamber.  Typically, the chamber is covered with a clear panel and the liquid is added through a small hole until there is no more air in the chamber.

I assembled the needed equipment and checked a couple of the chambers.  The valves and top cover are sealed with Vaseline.




The subsequent calculation yielded a compression ratio of a little under 9:1.  However, I had some recollection of having had the head shaved back in the 80s.  Some more research revealed that the stock head for this car should have had an overall thickness of 3.300 inches.  Mine measured about 3.290", so it appeared that I'd had about 0.010" taken off.  Working backwards, this implies that the stock car must have had a CR near 8.75:1.

I decided I'd like a CR of around 9.5:1, and calculated that an additional 0.030" removed from the head should deliver something close to that.  I briefly mulled doing the shaving myself, but it's a little big for the tooling I have, so I resolved to send it out.  Besides, I certainly didn't have the equipment to do a three angle valve seat grind, so the head was going to have to go to the shop anyway.

Before hauling the head to the shop, I took a little time to address a few small mods to the ports and the combustion chambers.

The exhaust ports on this head have a sizeable boss in the roof of the pocket above the valve.  The boss is to support the lower end of the valve guide.  That feature offers some restriction in flow, and since it is larger than it needs to be, it's a common mod to reshape it to reduce the restriction.




The exhaust ports also got some minor reshaping to maintain a relatively constant cross section through most of their length.  This little template, which fits the port opening, was used to determine where to grind.  The idea is that the template should be able to pass through the length of the port.  Something similar was done for the intake ports.  The amount of material removed was pretty minimal.




The only other grinding mod to the head was to blunt the "eyebrows" in the combustion chambers.  These are sharpish "creases" on either side of the spark plug opening, and also on the opposite side of the chamber.  These can reportedly be hotspots in the chamber and lead to premature preignition.




So, it was then time to get the head to the machine shop.  They would need the valve guides installed to to the seat grinding.  I opted for bronze guides because I like the color.




I consider my Triumph Workshop Manual an indispensable resource, but have learned that some of the specifications in it shouldn't necessarily be regarded as gospel truth.  For example, the WSM lists the length of the valve guides as 2.72 inches.  The stock guides on my car were much shorter, and the intake and exhaust were different lengths.  The replacements I bought matched the stock ones.  Also, the protrusion of the guides above the top of the head face is shown in the WSM to be nominally 0.750", but my stock guides all had a protrusion of about 0.630".

So, I departed from the WSM when I installed the guides.  The first pic shows the drift I used, and on the left, a collar to limit how far the guides got pressed in.




Because of the valve stems being out of spec, I also ordered new valves.  The exhaust valves are stainless steel.




I checked the valve stems in the installed guides and found that they were all very tight.  I fixed this by hand reaming the bores.  Inexplicably, the inlet valve stems are about 0.001" larger than the exhaust stems (the WSM has this correct), so it really needs two different sized reamers.  The closest I could come was a 0.3120" reamer for the exhausts, and a 0.3125"for the intakes.




Then, it was off to the machine shop for the shaving and the seat grinding.  Turn around was really fast.




At this point, since I had all of the chamber "cc" stuff handy, I checked all of the chambers.  They were all within about 0.3 cc of the same volume, which is less than 1%.  Since this will be a street car, I didn't stress about the variation.  The new compression ratio calculated to be a shade over 9.5:1.

The newly machined surface of the head looked really good.  Unfortunately, it made some of the other machined surfaces look a little shabby by comparison.  The manifold mating surface had dings, scratches, and dents that were suddenly bothering me. 




Since it is a much narrower surface, and I've never tried it before, I decided to face that side of the head. 

 


In anticipation of final assembly, I checked out the original valve springs, and here is another place where the WSM let me down.  It appears that the WSM has the free length specs for the inner and outer springs reversed, and the installed length of the springs didn't match my head, either.  Looking past that, I measured the "seat" load of the springs, using an impromptu lever arrangement.




Given the unreliable length specs, it was a little hard to know what the load really should be, but I concluded that the measured load was low for both the inner and outer springs.  So I ordered new springs.  The new springs will offer quite a bit higher load than the old springs.  Total seat load form both springs will be about 50 pounds.  This is still low my modern standards, but about 13% above the WSM spec.  Most of the extra load comes from the new outer spring, which has thicker wire and fewer coils than the original spring (and is wound the opposite way, oddly).




So, assembly time, but first, lap the valves.  This ensures a tight seal.




There are over a hundred parts here...




...but they all found a home.




And some paint to make her look pretty.




One of the three valve cover studs had long ago been replaced by a long bolt, so this was my chance to make it right.




And then to take care of a couple of details that I almost surely would space off before first startup.




On the shelf until I'm ready for it.




A lot of steps in this project, and a lot of repetition.  Also, one of the more expensive ones at around $350 for the valves, springs, guides, and the machine work.  Can't wait to install it.

Comments to Ed at elhollin1@yahoo.com

To my other GT6 pages