Not what I was expecting to find?

SteveA · Apr 8, 2014

Yes it was a joy to ride Chris.....not sure it stopped at 6,600 though ;-) and to be honest as a 750 it shouldn't have.....

if you look at those pistons they have been machined not only for vavle cut outs, but for squish, which will have reduced the distance between the ring groove and the top of the piston and that dodgy valve cut out.

Have you made 100% sure there are no witness marks from piston contact with the head? Whatever pistons go in will need careful set up and machining.

Have sent email to Norman asking details of his JE pistons....he might have some 74.5s

Matt Spencer · Apr 8, 2014

Was the missing piece spat out , or present ? .

looks carboned up on the break , so was displaced for some time .

Wondering if it got blunt force trauma at some stage . dropped on concrete or otherwise bludgeined . :?
If the others scrap , you could see if its coming of too . Or clean & inspect both microscopically or Zyglow . :?

johnm · Apr 8, 2014

I read carefully but couldnt see what your CR actually is.

31 degrees advance is a lot for a higher compression bike. Dunstall for example recomends about 28 29 advance for a 10.5 CR 750.

On my 500 at 10.25 I use 28 deg with 97 octane gas.

I used GPM pistons for more than 5 years in my 500 race bike but your problem here could be the narrow lip between the valve cut out and the edge of the piston.

Personaly I think your ignition is too far advanced.

I could suggest to read the Gordon Jenning spark plug tuning articles on how to set your ignition timing using plug readings - and take a look at the plug on the side that didnt fail.

Or did both sides fail? That would be very strange

Chris · Apr 8, 2014

Hi

First off, I didn't say it stopped at 6600 Steve just that it revved easily to that

John Both sides failed. I will dig out the spec & see what the compression ratio was.

I think everyone is correct & that the ignition is to advanced.
I also believe that the valve pocket enlargement ends up far to close to the edge of the piston. All the bits were spat out.
luckily cast pistons as the parts are soft & did not damage the head too much.

Steve only does 74mm so resleeve new pistons.
Contacted Norman so we will see what he has to say. Spoke to Norman he runs 28 degree on a Rita. Also informed me that Andover stock the plus 60 pistons (GMP) He suggested that alloy rods & 3 piece crank may point to the pistons touching the head. However he believes the whole of the crown would have come off. There are no witness marks to say the pistons have been hitting the head.
Possibly a bad pocket job? Poor choice of ignition advance. Really should have stripped the engine instead of using it for that last meeting. Ahh the benefit of hindsight.
Asked Comnoz for the JE piston spec.

Chris

frankdamp · Apr 8, 2014

When I blew the engine on the #2 prototype, it was a lot messier. At about 103 mph on the Mira track, a link in the final drive-chain failed and the chain came off. I just heard a bang followed by just a rushing wind noise. I was conerned about the Aston coming up behind at about 140, but managed to get off the banking before he got there.

When we took the engine apart more than it had done by itself, we foud wreckage that showed the rpms had gone so high that the valves in one cylinder had floated and been hit by the piston. The piston had cracked right down the skirt and jammed in the cylinder. The con-rod had just ripped the wrist pin through the piston, then had fallen over and punched through the case.

Good for a boat anchor!

SteveA · Apr 8, 2014

Chris said:
Hi

First off, I didn't say it stopped at 6600 Steve just that it revved easily to that

John Both sides failed. I will dig out the spec & see what the compression ratio was.

I think everyone is correct & that the ignition is to advanced.
I also believe that the valve pocket enlargement ends up far to close to the edge of the piston. All the bits were spat out.
luckily cast pistons as the parts are soft & did not damage the head too much.

Steve only does 74mm so resleeve new pistons.
Contacted Norman so we will see what he has to say. Spoke to Norman he runs 28 degree on a Rita. Also informed me that Andover stock the plus 60 pistons (GMP) He suggested that alloy rods & 3 piece crank may point to the pistons touching the head. However he believes the whole of the crown would have come off. There are no witness marks to say the pistons have been hitting the head.
Possibly a bad pocket job? Poor choice of ignition advance. Really should have stripped the engine instead of using it for that last meeting. Ahh the benefit of hindsight.
Asked Comnoz for the JE piston spec.

Chris

Funny that Norman told me to set mine at 31 and put a pointer on my belt drive cover with the plley marked at that, and when I said Maney recommends 29 he said 'I don't know why?' however, that is for a compression of low 10s, what do you reckon this motor was Chris?

Matt Spencer · Apr 9, 2014

Probly got the Wrong Plug . !

The GENERAL THEORY is that you back of the ignition , you loose Horsepower . IF yr to far forward or the plugs to soft ( same thing )
Itll Round off the nice square edges - on the CENTER Electrode .

pretty much .

Thus in the olde narrow range plug days , the intrepid machanics fitted the Soft Hot plugs for Warm Up - to obiviate Oiling . And CHANGED ALL THE PLUGS to the HARD RACE PLUGS once the Engine was Up to Temperature .
To some extent these modern wide range things have changed all that . But if your running ' ROAD ' plugs on the Race Track , youll get consequences of that nature . Pre Ignition . Spark Collapse and Delay , etc & so on .

If your getting the Throttle where it Should Be .

johnm · Apr 9, 2014

Chris said:
Hi

John Both sides failed. I will dig out the spec & see what the compression ratio was.

Chris

Both sides failing is weird. I suspect they didnt go at the same time but the bike only suffered major power loss when the second one went.

Dont understand what you mean about digging out the spec. No matter what the vendor says the CR might be for a given piston the only way you will know for sure is to measure it.

You will get a different result depending on the base gasket thickness, if someone has decked the cases (hopefully on a race bike this was done), the head gasket type, the valve seat conditon(How pocketed the valves are), the actual valve type used, has anyone planned the head, any machining that has been done on the piston to clear valves, valve pockets, any changes in combustion chamber shape, rebore on the piston changing the swept volume. All of these things will change the CR. The only way to be sure is to measure it.

cheers

John

SteveA · Apr 9, 2014

johnm said:
Chris said:

Hi

John Both sides failed. I will dig out the spec & see what the compression ratio was.

Chris

Click to expand...

Both sides failing is weird. I suspect they didnt go at the same time but the bike only suffered major power loss when the second one went.

Dont understand what you mean about digging out the spec. No matter what the vendor says the CR might be for a given piston the only way you will know for sure is to measure it.
The only way to be sure is to measure it.

cheers

John

True John, but going to be tricky to do now....and probably inconclusive.....and in any case more relevant figure now will be to measure the new achieved figure after setting up squish and valve pockets.....

I am not sure how 'decking the cases' translates, what do you mean by that? I would not want to take material off of the cylinder base joint when I had just relieved the cam follower tunnels for high lift cam clearance?

I probably will take material off of the cylinder barrel top face....

johnm · Apr 9, 2014

Decking the cases means checking and machining if necessary that the top of the crankcase is exactly parallel and square to the crankshaft. The aim is to get the bores exactly square to the crankshaft. Sometimes the bores are not parallel as well.

If you get everything set up exactly then friction will be minimised.

Nater_Potater · Apr 9, 2014

As the general consensus is overly advanced ignition timing, have you verified your timing marks with a piston stop? Mine was off almost 4 degrees! If I time to those marks, it runs like a dog. You could have the inverse, and be running substantially more than what the marks show you. 'Probably remedial, but I had to throw it out there...

Not what I was expecting to find?

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