OOZING SULOTION
Many moons ago, Stephen Herbert rebuilt a Royal Enfield Constellation. Over time, some oil seepage became apparent from the 700 twin’s top end. Time to experiment with gaskets…
Connie had a leak. OK, she’s an Enfield and they all leak oil don’t they? Well not necessarily, but this wasn’t an ordinary oil leak. It was much worse – a leaking cylinder head gasket. Or possibly two. I started noticing it late last year (or even before that if we’re being completely honest) but at the end of the riding season I put the Constellation away without doing anything about it.
Then came the winter lockdown and the threat of boredom. Lacking any other project that I could progress, I decided to investigate. I took inspiration from an article in these hallowed pages by Oily Boot Bob, which involved modifying the standard copper head gaskets to improve the seal. And while we’re on the subject of these hallowed pages, Connie’s rebuild was featured in this very journal a few years ago, starting in RC131…
Unlike its smaller, single cylinder siblings, Constellations don’t have spigots to locate and help seal cylinder heads onto barrels. Instead the big twin engines rely on two dowels per cylinder for position and large flat copper gaskets to seal between the flat tops of the barrels and the flat bottoms of the heads. Consequently, combustion pressure is contained simply by five, torqued-down head stud / nuts per cylinder and very large gasket areas. Because of the lack of spigots to contain direct pressure and the massive area of the gaskets, leakage under load is almost inevitable. ‘They all do that Sir…’
I was first made aware of Connie’s problem by a short, loud squeaking sound when firing her up from cold. I’d been haunted by this noise on a Crusader in my yoof. OBB’S solution consisted of
reducing the gasket area by removing metal to narrow the sealing area, thus increasing the pressure (load per unit area) on said gaskets for the same standard combined tensions in the studs. Greater clamping pressure on the gaskets is more likely to provide an effective barrier to combustion pressure.
There’s a pic somewhere here showing one modified and one standard gasket sitting innocently on Connie’s barrels to illustrate the point. Many thanks to OBB for sending me photos of his gaskets for me to copy!
Armed with all this information I commenced dismantling Connie’s top end. Although I didn’t need to, I removed her barrels too at this stage to check there was no wear in the piston rings and bores or the bottom end since the rebuild a couple of thousand miles ago. All was well!
OBB suggested modifying the gaskets by one of two methods he’d tried; tin snips or saw, and recommended the latter. Lacking the kit to do this neatly, I instead opted to use tin snips plus a 6mm drill for the tight inside radii, which all seemed to go well. After cutting, I carefully filed off the drill burrs. To make the job easier I annealed them first; heating to cherry red heat and quenching in water. It’s probably worth explaining at this stage that this solution won’t work if you’re using composite gaskets, cos the heat and tin snippery would destroy them. Also worth a mention is that Hitchcocks sell two different thicknesses of solid copper
gaskets; ‘thin’ and ‘thick’. Mine were the latter, about 1mm thick.
I annealed the gaskets again after surgery to ensure maximum compliance with the mating surfaces. I also cleaned up the sealing surfaces on the undersides of the heads, using emery cloth on a piece of scrap kitchen worktop granite. Don’t try this on actual kitchen worktops…
Impressed with my work, I wondered how much metal I’d actually removed and, by inference, the likely increase in clamping pressure. My wife’s digital kitchen scales told me I’d removed almost 50%, so clamping pressure should be almost doubled.
I proceeded to reassemble Connie’s top end, using Blue Hylomar on both surfaces of the base and head gaskets. I torqued the drive side head. No problems. Then the timing side…
…and horror of horrors; one of the nuts wouldn’t tighten. I feared that its stud had stripped the thread in the crankcase!
Much swearing ensued, followed by denial (‘It’ll be ok, there are another four studs and they’re tight enough’) but common sense prevailed. So I stripped it all down again to investigate the damage. I was right; the thread in the crankcase was ruined.
The obvious solution was a helicoil insert. Apparently another solution is to use a Tiger Cub stud, because that has a larger thread on the business end. I had some experience of helicoiling with stripped spark plug holes on a Lomax I built years ago, but Connie’s predicament seemed more complicated. How to ensure the clearance drill would go in at exactly the right position and be precisely vertical? How to ensure the tap also went in vertically? How to get sufficient swing on the tap with all those other studs and the other cylinder in the way?
After a lot of head-scratching and helpful answers on the VMCC Cheshire Cats Whatsapp group, I devised the following procedure…
To ensure correct positioning of the clearance drill, I first created a slight countersink using a centre drill of the correct dimensions to pick up on the original hole position. To ensure a vertical hole, I made up a Heath Robinson jig using a piece of scrap alloy plate, two tubes and some blue insulation tape. The tubes lifted the alloy plate over the inlet tappet and created enough height to ensure verticality. The blue tape was needed on the studs because the threads are rolled (ie. the threaded end of the stud was of a slightly larger diameter than the shank). I’m told that any colour of tape would work equally well…
To get the tap in successfully I had to remove two adjacent studs, saw ½” off the handles on my smallest tap-holder, and I used a square block of steel once the thread was started to ensure it was vertical in two axes. The insert was very easy to install, but beware the kits you can buy. These typically come with a set of inserts whose length is 1.5D (1.5 times thread diameter). I don’t believe this is strong enough so I ordered a few 2.0D ones for good measure. The tang was easy to break off, and I retrieved it with a pair of tweezers. (Keep that information to yourself please!)
After doing all this, reassembly – reannealing the gasket and re-coating with sealant – was quite straightforward. My concerns about the position and
verticality of the stud proved unfounded as the barrel slid down over the studs.
All the nuts / studs torqued down nicely and I was then able to install Connie’s headsteady, massive steel and alloy parts that effectively bolt the heads to each other and to the frame tube. Then I could refit the oil feed pipe to the rockers, plus the carburettor and exhaust.
On the first dry day after finishing the job, I fired up the Constellation. No nasty noises on startup so I left her running for 10 minutes or so to warm her up. Then back on the bench to re-torque the head nuts. All good, so I refitted the tank in readiness for a summer of Connie riding!