Hitchcocks Forum

My 612 bullet has always been noisy/clattery. I had an issue while away on a tour in Spain last month where it started intermittantly detonating on a long, steep motorway climb out of Santander. It eventually seized (not an hour off the ferry, you can imagine my face!). It freed off quite quickly and has done 1,000 miles since, I added octane boost to the fuel for the rest of the trip and had no further incidents but oil consumption is definately up.

I'm going to do a teardown this weekend and check the condition of the piston/bore. Best case a cleanup, new rings and a hone. Worst case, new piston and re-bore. Obviously dialling in the timing and fuelling is an ongoing process.

In any case, I've heard from a few places that the sharp edges around the valve cutaways on the piston crown are unhelpful in this respect and that radiusing them off can help reduce the tendancy to detonate.

Any suggestions as to what's best to use? I was thinking of roughing in with needle files and/or a rotary grinding tool then moving up the grits with wet and dry then finishing with polish. Someone them put the fear in me by suggesting I could leave abrasive particles embedded in the aluminium from the wet and dry/rotary tool.

Comments/suggestions?

Bonus pic of Bullet in the Picos de Europa, demanding/intense but perfectly surfaced switchback mountain passes for which it is perfectly suited. Didn't even get into 4th gear most of the time, it was the motorway that it didn't like.

Using a needle file will be fine. As far as polishing goes, anything will be better than the rough carbon surface you'll probably find when you strip it. If you use abrasive cloth or paper, just use good quality stuff that doesn't shed much.

I never fit parts straight out of the box; I always tend to smooth stuff off to get rid of any rough edges from the manufacturer's process. Having done that, I often wash engine parts in hot water and soap after working on them, to get any debris off them. I then make sure they're perfectly dry and use a water dispersant spray to keep them from rust or oxidation.

Smoothing off the sharp edges sounds like an excellent idea. If you're worried about the use of abrasives, normal clean-up procedures ought to work, and it's not as if the piston crown is meant to be touching any other parts of the engine. If you're handy with a set of engineer's scrapers or a small tungsten carbide burr in a die grinder or Dremel, no abrasives to worry about.

You've probably posted the specs of your engine before, so please forgive the usual questions.

What compression ratio is your 612?

Maybe a 0.5mm spacer under the barrel?

Is your inlet cam timed on the dots or retarded one tooth? That latter helped reduce pinking on my AVL hybrid which had been boosted to 10:1 compression.

Slacken off the ignition a couple of degrees?

Points, Boyer or crank-mounted CDI ignition?

Or, dare I suggest, fit a magneto with manual advance and retard and a nice little lever on the handlebars?

A.

Calculated compression ratio as 10.6:1. Has a 3mm spacer under the barrel. I tried the inlet cam retarding thing and it made no discernable difference so I put it back to standard timing. It's a crank mounted electronic ignition.

Well,on teardown, it'll be a new piston anyway. I think the bore will do, there were marks but I ran a hone through and while you can still see them, they can't be felt with a fingertip or fingernail.

Here's the damage...


As well as the seizure you'll notice the piston has cracked through the gudgeon pin boss.


Interestingly, when you look at the underside, none of the heat from the seizure has transferred across the crack. This suggests to me that it seized because of the crack springing the piston skirt out rather than cracking because it seized. The crack must have been there before the seizure occurred or there would be scorching both sides of the crack?


There are also two cracks in the ring land area between the two compression rings.


There is no evidence of nibbling/pitting on the piston crown like I'd expect with detonation.

I'm not sure but I wonder if one of the big backfires it was doing before I got the timing dialled in caused a crack to start and it's been slowly propogating until it finally let go in a bigger way.

I'm astounded it kept running for as long as it did. Look how much play there was on the piston! Miraculously, there doesn't seem to be any damage on the small end.
Link to short video

That looks like a manufacturing fault to me.

Looks like a Vauxhall Diesel piston from the mid eighties
The cranks were bulletproof

No help here

Radiusing edges on piston crown seems like a good idea we used to radius both crown and skirt on two strokes back in the day but have never done a four stroke

Cheers Dai


EDIT

Also used to line up gudgeon pin clips with gap either at top or bottom

G'day SW
Sorry to see such damage from an overheat- the ring land distortion, skirt damage location, and heat colouration is clear in the pics.

A few questions if you don't mind (to avoid this again):
What was the piston to bore clearance?
Which cams are you using? # 200180 or #90023?
What were the WOT AFRs at 4000rpm and 6000rpm?
What were the timing amounts at 4000rpm and 6000rpm? Does your control box allow a custom curve?

Kind regards
Paul

Without being rude, I doubt that very few, if any individual Bullet owner has those figures for their bike.

Kinda lucky it nipped up before the piston let go.
Those cracks between the rings are weird, not seen that in isolation before.
👁👁

Haggis wrote: ↑

Mon Oct 25, 2021 11:15 am

Kinda lucky it nipped up before the piston let go.
Those cracks between the rings are weird, not seen that in isolation before.
👁👁

It did another 600-1,000 miles after it seized, all on mountain passes in Northern Spain!

Blown265 wrote:G'day SW
A few questions if you don't mind (to avoid this again):
What was the piston to bore clearance?
It was supplied as a complete piston and barrel kit from our hosts so I am working on the assumption they have bored it to the specified 0.135mm to 0.145mm.

Which cams are you using? # 200180 or #90023?
90023

What were the WOT AFRs at 4000rpm and 6000rpm?
Not had it on a dyno. Probably varies with the phases of the moon because it has a stick in a hole aka Amal MK1 Concentric fitted

What were the timing amounts at 4000rpm and 6000rpm? Does your control box allow a custom curve?
It's a controller from electrex. It's set to give 32 degrees BDTC fully advanced and has a fixed curve.