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[adamscarborough]

A combination of new parts in my Bullet 500 is causing the inlet valve to clash with the crown of the piston.
Please read the end paragraph and question if you don't read the rest

I have fitted the Samrat improved rockers and new alloy barrel and high performance cams. The piston is the original, the push rods are the original ones. the head has been fitted with the enlarged inlet valve and skimmed 0.005in.
Between the crank cases and barrel is a standard gasket, the head gasket is 0.5mm copper.
With both the ORIGINAL cams and the performance cams, as the piston comes to TDC and the inlet valve starts to open the two clash. Even with the push rod to it's shortest and seated fully, they immediately clash, the only way now to allow the piston to reach TDC is to undo the 4 rocker nuts and allow the whole rocker assembly to lifta couple of mm. With the push rod at this short length it is totally loose and not held in position.
The valves aren't sticking. I've not accidentally fitted those valve caps that the original valve have and the timing is correct.
The exhaust components all adjust and work correctly.
with the original barrel and original rockers the engine does rotate, but I suspect the velve is VERY close to the piston.
I have measured the height of the new inlet rocker and it is 1mm shorter than the original.
The new barrel measures the same as the original, but in situ. possibly has a reduced height of 0.2mm, for whatever reason.


Because of the enlarged valve diameter it misses the cutout in the crown of the piston, and the cutout may be too shallow anyway. Can I safely enlarge this cutout (by hand), closer to the side of the piston and a little deeper for safety?
I have already spent this years budget on parts! and I'm can't afford to buy a new piston only to find the cutout is just the same.

[adamscarborough]

If you look closely at the piston photo you can see where the valve touches and has "cleaned" off the carbon.

PS it's a shame you can't go back and edit your posts as they never come out how I intended!

[Revband]

A lot of changes all done at once often end in tears, but yes you can increase the diameter of the valve cut out in the piston, don't forget to allow enough for the piston to clear in every possible movement, don't go deeper unless you really have to that is a bit risky?, Reassemble the head with blue tack or similar in the piston cut out oil the valves to prevent sticking turn the engine over then remove the head again and check the blue tack to see if you have a safe clearance.

[Bullet Whisperer]

I have encountered those large inlet valves and they are bigger than is ideal. You could reduce the valve diameter by possibly a couple of mm, to help it clear, raise the barrel and / or head height a little [rather counter productive], or, best of all, retard your inlet cam - a favourite trick of mine.

[stinkwheel]

Doesn't look like there is much in the way of deck clearance there (height from edge of piston crown to top of barrel)? Then you're using a half-thickness head gasket. Is it not going to have a very high compression ratio at that?



Others may correct me but isn't the standard 500 piston at enough risk of grenading itself under normal conditions, never mind with increased compression and a larger fuel charge? If it were me, and I couldn't afford a more robust piston (regardless of clearance), I'd be considering steps to reduce the compression a bit, this will also cheaply and simply help alleviate your clearance problem. So maybe a thicker head gasket and an extra base gasket? Or even a compression plate under the barrel?



Admittedly this wouldn't be making the most of your additional tuning mods but if you've blown your budget for the year, those bits will still be there if you want to do more in future.



One other thing, just from looking at your pictures. The recess in the head seems quite shallow, although this could be an optical illusion. Is it deeper than the spigot because it looks like it might not be? Are you the first person to have skimmed this head?

[Leon Novello]

Some things stinkwheel is talking about.



poems by John Dryden
>

[Revband]

One thing to clear up here, the head/barrel compression seal on these engines is formed by the joint between the head recess and the barrel spigot if the head has been skimmed? Then a thicker gasket will be needed to prevent oil leaks unless the recess has been skimmed by the same amount.But the head must bolt firmly down on the spigot.

[ChrisD]

Hi all.
As Adamscarborough indicates, even with the original cams, the new inlet valve contacts the head. It may simply be that two 0.5mm copper gaskets are needed. Maybe that will give the necessary ~20thou clearance. These bikes are designed for a thicker composite gasket anyway – about 1mm (~40thou) thick.

I wouldn’t use ‘blutack’, I’d use something stiffer and less sticky, like playdough. The equivalent to Blutack out here in South Africa (Prestick) reacts with the oils on the piston and makes a very hard-to-remove sludge.

Cheers, ChrisD

[stinkwheel]

They do plastiguage that covers 20-40 thou". It's designed for use inside engines to measure clearances.

[Revband]

Indeed they do, but I just thought other things would be more easily to hand,;-)