Royal Enfield Bullet 612cc project

[struan80]

It's good putting a face to a name.

[stinkwheel]

Put another 600 miles on the bike at the weekend. Protracted A-road/motorway section and some twisties in the Scottish Highlands complete with luggage. including a 7 hour run back down from Royal Deeside.

The rack held up well, I think I'll put one cross piece and a couple of leather straps on. I'll als re-infirce the two flatbar pieces to stop them flexing side to side. I think I might just put a bit of a lengthwise curve in them which should make them much more rigid.

The bike will happily sit at 65/70 on everything but a steep hill, it never dropped below 60mpg.

I've pulled the top end off with a view to both having a look and to removing one of the compression plates.

This is as clean a burn as I've seen on a Bullet piston, pretty happy with it. The mark on the crown is where the dead-stop tool marked the carbon deposit them I picked at it with an oily finger. It's a bit on the lean side I think. I'll try upping the main jet.

https://lh3.googleusercontent.com/pw/AM-JKLW2wGmbNiaoIBhbHC6-brHynfS_G9hTyNRaZ-TsBZANaNO80VK5W6u4hO5SZfUkwVUy-K5KCAleW1ukpvU-otaEbuRB8U1rhI47ZDUzA3cha_YW1f3tbXlpVSuEFjZFkmg2Q58pGLvDp5gHbNcY8Q1E=w1576-h886-no

Again, as little blowpast as i've seen on an enfield. It would be terrible on a Jap bike but there's a hell fo a lot of oil hanging about in this motor (no valve stem seals etc). I think a "hard" run-in was the way to go.
https://lh3.googleusercontent.com/pw/AM-JKLWqKH-ykArXEfYLUrg7paQ5iEBRZdz2Wk7Rl-kp-rMKPlm_B6e1FfgK1oQOHa6X1jWO7ONx172r1d1xGRS1P376HWmMB8-WZyRjuAlVZgIOf2R5oKBJEC_MWnIBzR79QCViO68_Fvw3Tw1ThhIBnrE0=w1576-h886-no

I've calculated the compression ratios and if I remove the 2mm plate and leave the 3mm one in, it should make 10.6:1 compression. If I use the 2mm plate by itself it would be 11.6:1 which is a bit on the steep side for road use and road fuel.

Job for the weekend is to reassemble with blu-tak on the piston crown to check the valve to piston clearance is acceptable with the 3mm plate then assemble with new gaskets. I've managed to get hold of a composite gasket with a proper fire ring in place of the copper one it currently has.

Turns out the adjuster on the exhaust pushrod was also slightyl bent but this didn't cause an issue with clearances because it has a "cup" on both ends. Easily replaced, I have a spare from the previous bent rod. It's no longer backfiring on startup so hopefully that issue is past.

Couple of other housekeeping jobs to do including making an alloy deflector plate to stop the breather output flinging up off the chain and all over the bike. Also to re-fit a shorter rear brake arm to stop it camming-on.

I have some diferent capacitors to try on it to see if they'll store a bit more power, if not, I may go back to battery. It should fit int he "spare" toolbox.

The plan is to go touring in the Spanish Picos on it at the end of September. It's shown it can do it now and I'll hopefully have a bit more poke to get up to a cruising speed of 70 and a little in the back pocket for overtakes. If I want to have a higher cruising speed, I'd have to start messing with the primary gearing because the transmission sprockets are as tall as they'll go.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[stinkwheel]

Checked valve to piston clearance by means of applying blu-tak to the piston, re-assembling and turning the engine over then measuring the squished bit.

5.5mm. Which is a country mile. Minimum is 1.3mm. This engine would be safe from dinging a valve even with negative deck height. Which is reassuring.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[virus]

[stinkwheel]

[bladerunner]

Nice to know its working well and doing its thing! I've done a few hundred miles on a borrowed Derv bullet this year and it's way off my usual litre size jap sports bikes or small 80's/90's 2 strokes......however I must admit it's been ace fun in its quirky kinda way and I'm envious of your extra HP.....I think torque wise I may have you beat, but handling wise I'm actually rather impressed with the old Bullet for what it is!! I'm managing just over 200 mpg so given the long range tank John O groats to land end is doable with 50 miles or so left in the tank Wouldnt actually know for sure tho as it last got fuelled in 2018 and has only covered 1000Km so far and I'll leave it another couple of hundred more before I top up again!!
____________________
Current bikes...cbr929, KDX200's, Rd125lc mk2, RGV250's

[stinkwheel]

[stinkwheel]

Been quite some time since I updated this. It's been eventful!

The Picos trip in 2021 was a qualified success. Qualified in that the engine seized about 20 miles out of the ferry! It freed up again and sucessfully completed the tour, covering some epic mountain passes. However by the time i was getting back on the ferry, it was making some horriffic noises and burning an ourageous amount of oil.

On teardown I found this:
https://lh3.googleusercontent.com/pw/AL9nZEUOXINFgoL26aCxSnHbkBWeZPaEsDY3h5rgbj8r7yvBFk3AI_GIe88Tq07DfwOp4JeAPkQGFfC6TtXdfw4H0Cn1FJyrlhlIiK5puvYi_RFeEDBnPXTV0y3F9eevRe4sqNqROhwHdLa9ts4BAFs7iez7=w1378-h776-no

https://lh3.googleusercontent.com/pw/AL9nZEVBT2zWXc5Q6VwtL4wXLHECd05fRKbgGtxg7q6aGD0XGC9MEXgugv8kcEBi-INpRNowYEWBfI2Mi2ysml1MDV_lcPT7tK_5yUx0mEDNabIyf3VsozMqdHYfFinQUW-c9_ExlEOZ_2AAREyf9gI0ZXsi=w1378-h776-no

https://lh3.googleusercontent.com/pw/AL9nZEWsbMyochbvGiSGvUQkuULveJiafVMlocP33s9WQBwV_jdSdCYcQFOyL2fWaNC9fSR0NOVRAbrVqQ-lsNVlLNULv8BHvCAU94TGGfYa0dlc-AqeMcIZijVmqq-wmxf3-l9VScvFDdx73QAZ7B0pYbIU=w1378-h776-no

Video

Umm, yeah, there was a good 4mm of vertical play on that piston. Looking at the last picture, the seizure seems to have been a result of the piston cracking, noth the other way round. See how none of the heat was transmitted across the crack. Scrub one £200 Accralite forged piston!

It turned out Hitchcocks had a stock of barely used pistons they used during development so I got one of these. I polished the crown and eased/rounded any of the sharp points. This will probably have decreased compression a bit but should have benefits in gas flow and removing detonation points.


I also decided the electronic ignition was possibly a bit iffy (not so certain now) and put the points back on with a new advance/retard mechanism. I strobed this up to give the correct advanced timing of 32 degrees and confirmed the advance was correct.

https://lh3.googleusercontent.com/pw/AL9nZEW6hQouDBgO40fINuAkZZ-3S_vJBdr5sZ7EZ1R5M9W5AFh7T0drKn8k_9uiOREPzV0DTwTTZOZhb7mXQEdDB3LP50Svf0HhYQhUXRmiZI5tUDx_6v5odYtfr1nRYtXOK34UiyA57vSIyjXxlq7ZZMdM=w1378-h776-no

https://lh3.googleusercontent.com/pw/AL9nZEW4PlwkGmnP3Iq5tYob_noEqGen1S0PVg0K2ahJXhQwekKoxB8MYoCOJx68TYWAuL42Yxu2PbbyS03Duyq25lGDCp7WqcNa7T86kgDLERFQC8YBAl28IeP5RY453v5EKZ9vhDWPJldggXzhg1h0V4Da=w1378-h776-no

Also fitted stronger springs to the auto-advance so it stays in the retarded position for longer.

https://lh3.googleusercontent.com/pw/AL9nZEV18zR5p61gA4KrtrDwbu1nB4QDuktbZ7qoqpPnUMh8aPkwteQYkoLuyecxVNj7UNNpZdkkcRo5ApBJ09jJbxRVtIy-k3NTM7v4aXnOA8_ajNuB-GyrOFX9mgJxXVK5prp84IWWpxaGV0LbkA3opG6u=w1378-h776-no

Also finally got my TM32 carb! Long story, came from the US eventually. Stuck in baseline jetting supplied by some other enfield enthusiasts.

https://lh3.googleusercontent.com/pw/AL9nZEUYZxp64BnnvRSyNf9fwl2xuJeLS6qtbU6SdZpSdcbcdqCTVWWKC9pjbfiKarPYkLAt6IzSCfKgaf-6gzAeD0iin3d7YxMS3DmX2MEPkdEKs5RfaEWKbpg-7AfOPWNWG4PjT7Mp4RGN92oFZlIM0-Q2=w1378-h776-no
https://lh3.googleusercontent.com/pw/AL9nZEWsq9jkhr0rKULxLdkPiVjVcj15dFnNl6-jVen5YkKk_OceBk1uQ5fvM59J0z7HEb55nnNMyXE5wHSTXE0PEZR5be-OKYnxHQpEKeYZefBGxvRZEIq2d9FUSEQZu8pgwAJuIrD6Sjsb96X5VGsDfVtA=w1378-h776-no

It starts great but pinks horribly under load. As a temporary measure, I retarded the ignition until it stopped pinking so I could run it.

The bike made it down to the Dragon Rally and back through some of the most torrential rain I've ever experienced so the wiring is good!

Then came yet another bent pushrod!! I finally got this one beat though. Which admittedly did go via making a set of 7075 solid rods (unbendable but double the weight).
https://lh3.googleusercontent.com/pw/AL9nZEX33YdEWl9YugaFTuPPXHVPUhSsbnDchCQp7U70lbXb7ij76ee48CT8RJoMKg2XHGvQncMV67hqLvrtDZNJYa40AVUxevUSxLarlouKkucEWrfalYcasOjlgH_JZGW2FrYSZLud-Lj8meu1HtsNzwVV=w1378-h776-no

https://lh3.googleusercontent.com/pw/AL9nZEXNm_e0XH_0JOjVgesuEblA_58tQkYDBOH9kLrDiDdYB8BL0WgsenBBom8jvzfJiyL4c2NRvzWlir_8mJl9vIsdbWmsot4dryKBXyGs7mruIx_mB0MgWCP_DpLI_NPjnfFrL3cRWzQ9iH-EfMzOpJUC=w1378-h776-no

On closer inspection of the cams, it turned out the inlet cam wasn't making continuious contact with the follower. The performance cams have a smaller base-circle diameter than standard and the follower was bottoming out on the crankcase 0.004" off the lowest point of the cam resulting in it being wacked open instead of lifted open.

https://lh3.googleusercontent.com/pw/AL9nZEWsN5r8v_WuNBy5wKw3XQBwKoYr0vG4K_g3loCq-5VD-y5fxnZ96p_aSexqZXaFlGFocsz12WU54BvoH0iWYGFJjlX_ykpbjkUHHbeNsArfMIMAkTEHgJySGkCaWL4xe40TfL5zT6K4sWJNGi4QWMHJ=w1378-h776-no

This was fixed by removing a small amount of material from the crankcase with a die grinder so the follower can drop further.
https://lh3.googleusercontent.com/pw/AL9nZEWs5F5zVwSFHavASNYQ8jkwJTYGBVZbfxoHE8eunh_tt4cdbf7gX3MBbkTR_-rb7j3NastztPNgyb_UkPjyqB1LWQLKmoYQaYmnaVFXmVY3yal-oWruHndUuuW_dOPfqUle3vBVeAgflO_CpVZhbibs=w1378-h776-no

It also became apparent that the fuel flow from the standard tap was woefully inadequate. i went full nuclear here and fitted a pingel tap which to quote my mate "Is like turning a bucket upside down". This will empty a full tank in 3 minutes.
https://lh3.googleusercontent.com/pw/AL9nZEXDj_n-9cHlX-5RecKHIIfv9jEH8ntrX7jtfEfWXYeEGyYnm61pIVd-ph9GNRKSx-2ENjBDgjV0yYPyKoqwDV-qOSGZppEI9MgR6bO1FZMHfGFputMDKbXJ-vCVe-A6FzpNxARNqXgdfLbOTMFy30Vw=w1378-h776-no

And that was another year gone. Put a few thousand miles on it and time for another summer holiday. We decided to do Northern Spain and Portugal again.

Would you believe it, got 20 miles from the ferry and it conked out, in almost the exact same spot!. This time though it was quickly traced to the battery lead having come off. Two days later it conked out again because it turns out the intermittant battery connection had fried the reg/rec and I had been running on total loss. No matter, a spare battery was procured and I just charged it every night. Fantastic tour of the picos and Portugal and the bike ran brilliantly.

https://lh3.googleusercontent.com/pw/AL9nZEXuQSbK2yqPQ5dKx8_hoAvIkrzDqBT3ZRXJeKuy2KmqNU3rEVjfAPJUWZwZu47wNinoFOHaascq0Eo7B-OpzPGog-oZE4VUFeATew4RiuZ0tBoth45k3sQFwD9QJ2cClGTAahxTHoxGCVh9d5JQfpFv=w1378-h776-no

Made sure the battery leads wont come off again!
https://lh3.googleusercontent.com/pw/AL9nZEW4iGHzdw1b3rhQARkDbr-d5PelGWIgV_it58ks0vfmXfxb4To0t2zjPmCBWBbBp5MfItiNCAt9PtmEdU4BMXBOh4dSikxCk3kaTtiuDlvVAstQJubmCshplP6x7w8Ui2jesUS4bf2nNjyRTMbuBfbY=w1378-h776-no

https://lh3.googleusercontent.com/pw/AL9nZEXGNLzXk2qSAAPgieoPMrZHrjk-itqOVCmUuUNz_SqLmpZabgN8s5XiQjivzFRXEaa5iLWdccbVpP2jzij3y0No2gEAxibkpBKZuOOX5zUrxQXIZiR83dVM8RLRFmwNkUnSp1U7Ah6pxMCCHVvSdIWP=w1378-h776-no

Which brings us back up to speed. The observant will have noticed I've been running it with the ignition retarded for the last year. Not ideal. I feel it is down on power from where it should be and perversely, it's hell of a latchy to start where it fires up first kick with it properly advanced.

So with the timing set where it's supposed to be, it's time to commence the enormous faff which is setting up the carb. There are 150 possible jets, only one of them is right!

Step one is getting the throttle positions marked and baseline it. Trouble is, it pinks under load anything over 3/8 throttle so i need to get that sorted first. It's fine up to 1/4 so the pilot jet must be in the correct ballpark. I've tried a larger and smaller one and it was worse.
https://lh3.googleusercontent.com/pw/AL9nZEW3jfkM0VyBUjC5LtD7zAnDIQOZo-t3pE2tTOljfniufhUnFFEyMdBJFIU0rOjKiVKNa2SI8VmyqM0XsO6NWMdtHyIOazM-Y-8pRkhr94a8s-1IQnm7pg2zoGPr08YKVNyp7SJv17VXGxCRmMHqDqdI=w1378-h776-no

Thing about Mikuni flatslides is they are a very known quantity and if it's lean between 1/4 and 1/2 throttle then we're looking at needle jet size, Jet needle taper and position and possibly throttle cutaway. Every performance bullet I've asked about is using a #4 slide so I'll stick with that. About to fit a P4 rather than P6 jet needle and a more aggressively tapered needle which is what a coule of Americans with "fireball" tuned bullets are using.

Once it's running nicely up to 1/2 throttle I can start dialling in needle position and main jet size. This will be a long-haul! Can't even dyno/plug-chop/GPS it until I'd got the pinking under control.
https://lh3.googleusercontent.com/pw/AL9nZEWH3W_kp4LJnYaCpYIFt6glfOs4tvVxCt7y2h2iOZd4M3PYIu4DdoBFxinLW2CSL0WmW35iAaX6GQE_9nQ1zVaJxZO8L2dBAg0QxKqnX6Cd8KwyqlWp7Yk2fm2LPR9MhFG8AeSxtFiFVVLMSddC44Ln=w1378-h776-no
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[stinkwheel]

Not updated this for a while.

Bike's been ridden a lot, there seemed to be a perennial problem with detonation. It seemed to be particularly fuel sensitive. Anyway, I pulled the top end off to have a better look and found one of the piston rings had shattered.

https://lh3.googleusercontent.com/pw/AP1GczNzFPip1MQhNkXDspxKezWW0ZspuJ-JcS3uQiCwn4ecRIWu10uHpkQW3nDn5j4dDKulEYQY0b1clu-s4eFTc4J6ecuHyDkWKDxU0La_56SNqPcdbTBzGaERab3FGpeMMmZyEUlY_n92IYa3tIS_N3QU=w1669-h939-s-no

So that was a fun day. Worst. Jigsaw. Ever.

Closer inspection of the head showed it was clearly leaking gas round the edge of the spigot and starting to "nibble" into the edges of the head. The head gasket fire ring was catching it but this seemed to be the source of the "pinking" I was getting.
https://lh3.googleusercontent.com/pw/AP1GczMqv8TGu3tl6VL9Ey6menNqFt5yPjukWwxttvCYJocF17lqLZEaOETzWErim-fdJrEBSzron6EJncHQdCoxe7pK5PCrBCdODvvqYnnJaqTGY1c9eFZbcOpmOHZ3YK6CXAST0vePQi1V-ogHaiY7GkRo=w1669-h939-s-no

https://lh3.googleusercontent.com/pw/AP1GczNu_heMiQe76GCbGK3kJTuS6gMBMKNjZPxxtZPGkTWaRyAmCch6unI8Zoc-cN8rvj0CjSIhekUOWKs55aFvFbjPV9_C9vCR74gDr6EL6Jzp6qEbbOmdp9oMeSBgRV8xmCd4r-J6ztILNZDIWpjv582K=w1669-h939-s-no

I set about measuring the spigot clearance and it didn't even register on plastigauge, I had to use greased solder in there to get a reading. It was coming out at 0.7mm. There isn't a lot of info out there but it probably wants to be closer to 0.05mm. ie a measurable gap but small as possible so when bolted down, the head gasket is fully compressed and the spigot gap is as close to zero as you can get it. The tiny gap will then fill with carbon deposit very quickly.

Having taken a load of measurements, I put the head in to my local machinist to skim off the excess. In doing so he found the head and spigot surfaces were not parallel. So he machined the rocker cover facing to give him a reference surface then skinned the head and fly cut the spigot recess to parallel then set the clearance.

While I was in there I also noticed the swirl pattern of combustion was very lopsided.
https://lh3.googleusercontent.com/pw/AP1GczMqv8TGu3tl6VL9Ey6menNqFt5yPjukWwxttvCYJocF17lqLZEaOETzWErim-fdJrEBSzron6EJncHQdCoxe7pK5PCrBCdODvvqYnnJaqTGY1c9eFZbcOpmOHZ3YK6CXAST0vePQi1V-ogHaiY7GkRo=w1669-h939-s-no

Not a clean or even burn. Not entirely surprising on such a big single pot motor with an offset sparkplug.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[stinkwheel]

You'll see from the above that there are two threaded holes in the head. One is the sparkplug, the other is for a decompressor valve. It's required because the kickstart goes through the clutch and it tends to slip when you try to push the engine over compression. However, I don't have a standard clutch and I can take it over compression without using the valve, so it's effectively redundant.

That presents the possability of screwing a sparkplug in there and twinsparking it. The standard plug is a B8ES, the decompressor will take a B8HS. It requires the recess the decompressor valve sits in to be machined out so you can get a plug spanner in and a small hole that bleeds exhaust gas from the exhaust port to the valve to be blocked up. I had this work done at the same time as the head skim.

Twin-plugging a points ignition single cylinder bike isn't as straightforwards as it sounds. You can't just use a dual output coil, they are designed for multi-cylinder engines so only one of the plugs is firing under compression at a time, the other would be a wasted spark. There's not enough "ass" behind them to fire both plugs at once in a compressed environment.

So we come to using two, single output coils. One of the enfield models does actually use this system but it's fired by a CDI so retrofitting that was not ideal.

So parallel or series? Assuming a standard 4 ohm coil. Series would see them experiencing a significant voltage drop on the coils. They'd be seeing 7v each and would be drawing 1.75A. Parallel would maintain the voltage at 14v but with a massive 7A which would cook the points.

One way to deal with this is to use two 6v coils in series (which are usually around 2 ohm primary winding). They'd be seeing 7v each and drawing 3.5A

Another way would be to use the 12V coils in parallel and using a ballast resistor in series with them to "fine tune" the voltage and current draw. This is what I did in the end because I'd already bought a couple of 12v coils. I worked out that by using a 1.6 ohm ballast (readily available for small tractors and plant) I would be seeing 7.8v at the coils and a 3.89A current draw which is high but possibly not too high. Were I to do this again, I'd probably go with the 6v coils in series for the first try.

I decided to fit a condenser across each coil and remove the standard one from the points backing plate.

https://lh3.googleusercontent.com/pw/AP1GczMwuYU65eb9hF-8zInbg-09Hs-ZYB-ec4inZTllO_YkaUOyd4T34qxREBrqSyBg_kZ67dP7oxNaRfOjaZsEWOV59yEKWDigQgSZeVhkk60_0hdpfpwORB-h5l2yWLyd3kNgC8QnxVMlWAFij1NKXmVgfw=w937-h785-s-no

The physical mounting of the coils went well, I made an aluminium plate to fit in the triangle of frame under the seat. This used standard captive nuts on the frame rail and was near enough to the standard coil mount I didn't need to mess about with too much wiring.

https://lh3.googleusercontent.com/pw/AP1GczMZb0SkWIffpYTeaoQqHX9aJ9CKYTPGaMVn9Jri2fyi-75Y2rkqMOG282B3C5ZFDxNKgwNfyE1B_5JS4icFovwWINTYFB4GW0JD_IKebqwyj-DwVCpZ79oEa6b8VkwBFIRL0AJB8T4xsKK_bG1Io7N7tA=w1669-h939-s-no

https://lh3.googleusercontent.com/pw/AP1GczOXH-4CQ4kKXplmoHH7VKcKtRA5VNOj--i_o_hKrXKXtC-L6kwTEvxaaa3i6Se4YVu4bRT9pbDtVECDXi9IB22AiWp4NREvjqFILoJvZ2vXKjGsT4UFcJjtuTsANuIW9t9jD3rPBMAEFw9Emyil9tkVhg=w1669-h939-s-no

Being aware I was operating the coils significantly below their usual voltage, I was a bit unsure if this would work. As it turns out, there was no issue. The bike fired up more easily than it ever has and rode nicely. As a precaution, I made up a loop of wire with two ring terminals which can be connected in to bypass the ballast resistor. This would work well as a "duress" starting system if the battery voltage was low. Applying it would up the coils to battery voltage and a significant current draw (which would cook the points if run for a long time). The loop could then be disconnected once the bike was running. In addition, disconnecting one coil with the loop in place would return the system to the standard setup to get you home.

https://lh3.googleusercontent.com/pw/AP1GczNCWCv4U0AU95EYh-9i5hDGDxaohEHFLTLOnCKaME-uAPSyZCFyVdiHHmNhVZBN0HvpkDa_JxPuhWKOa9oIqyjGrPQo8hRYvVALAIsDojRx3jEp01FiFiiMTJpE_XrBrDchK0l19KOmNSN4DA3Ok57Alg=w1669-h939-s-no

I've done a significant road test on it now. After about 100 miles, the engine started pinking when under high load. This is a known issue with twin-plugging big singles, the flame front moves a lot faster and it can be necessary to retard the ignition timing slightly. I duly whipped out the spanner and retarded the ignition by the width of a sharpie line.

This turned out to be the "secret sauce". The detonation disappeared and the bike did another 300 miles without another recurrance. It stanrts more easily, runs a lot better, pulls noticeably stronger uphill and seems comfortable with a significantly higher cruising speed. Sits comfortably at 70mph now and will pull up to 80 for an overtake*. Overall, a sucessful exercise.

*Remember this is a 1950's pushrod engine. The ability to do 80mph and not be concerned it's imminently about to explode is remarkable, this bike has been geared as high as you physically can without regearing the primary drive, it's up 4 teeth over standard on the front sprocket and will pull that gearing in top.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Pete.]

That nibbling is caused by the detonation. The fact that it's on the face shows that the face is exposed by an over-sized head gasket sealing ring I would say.

Advanced timing can cause a pre-cursor to detonation (by causing pre-ignition or pinking) but the detonation itself is the fuel charge firing uncontrolled often before the spark had occurred by excess head and/or compression in the combustion chamber and something to trigger it (glowing carbon, a sharp edge on a valve cutout on the piston, dirty/poor fuel or burning oil - that kind of thing).

Since your nibbling is around the edge of the chamber and not showing on the piston crown I would check your squish and sand off any knife edges on the piston crown/edges/ combustion chamber. Maybe re-machine for a little extra squish around the edges which will lower your dynamic compression a little and give a little more margin before detonation starts to occur.
____________________
a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[stinkwheel]

It's designed to seal with a spigot on the barrel against the head like some 2-strokes do, the head gasket shouldn't be doing anything significant to contain the cylinder pressure, it's mostly there to seal the oilways. Although this isn't what was happening.

https://lh3.googleusercontent.com/pw/AP1GczMsIIzyYDYHiyvzBCC4hNI0blqnfqu77LuxuPwhK8qxlcCikTDpFJqiiuCG6thRIjaf7Ibt2goAsng8fQ3Bop4vDVfc4YsLvXffRcS8RJIy7qI6RyMcFl0uxcvma3YHd-OQ69PT8MYcRpH8hL00rxfs9w=w1252-h939-s-no

In an ideal world, the spigot and gasket would clamp similtaneously, in the real world, a gnats cock of clearance is necessary unless you want it to leak oil from the head gasket, although some people do away with the head gasket altogether and machine grooves round the oilways for o-rings.

The "nibbled" area is outside the spigot and shouldn't have been seeing any gas at all. There was actually an 0.7mm gap between the spigot and the head. Creating a perfect pocket for detonation. The head is now skimmed to an 0.05mm spigot gap with the gasket fully compressed (confirmed with plastigauge) so that rim just inboard of the nibbled area of the head is all but touching the spigot. It'll have filled in with carbon by now.

Presumably, somewhere in the production run, the Indian factory lost sight of what the spigot was for, or the tolerances wandered. On a standard bullet with very low compression of 7.5:1 and a low state of tune, it makes no difference, a copper head gasket holds the pressure and detonation isn't even on the horizon. This engine is running at 9.5:1.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.