Not what I was expecting to find?

Hi

Stripped the 750 engine as it went ping on the back straight at the end of last season, lost a little bit of go, so I stopped it & sat out the last couple of races.
I must admit I was thinking a spring had gone.
I have heard of this happening but not to me.

spot the piston ring




That's a big valve cut out? And yes "both pistons"

The head should clean up (but will leave a fair few dinks.

GMP pistons 74.5

Any suggestions for pistons they were supposed to be Maney/Cosworth. The barrel is fine but 74.5

Doesn't look like I will be out to play this season.

Chris

Doesn't sound hopeful Chris, I thought that Maney used JE pistons to his design?

You were out when I tried to call back yesterday, lets try and talk next weekend, actually I am home on Friday too.

I was at Norman White's on Saturday collecting two sets of barrels honed for Jim Schmitt pistons! 77.5mm and 77mm, (so only any good with his rods!)

One of these sets was linered with RGM liners, £66 plus and fitted for £146 plus....looks like a good job...Norman was happy enough with using them in a race engine....probably the only way forward with a knackered 74.5 bore...

Will be starting my engine build very shortly. I will look at the pictures later...blocked from here!

Hi Steve

Phoned back 5 times Sunday!!

Spoke to Poppy this morning, so know you are in Italy. Have to speak next weekend, lots to do & getting nowhere.

ps Brands looks hopeful (not)

Chris

Chris said:

Hi Steve

Phoned back 5 times Sunday!!

Spoke to Poppy this morning, so know you are in Italy. Have to speak next weekend, lots to do & getting nowhere.

ps Brands looks hopeful (not)

Chris

Click to expand...

Chris,, I cant hear bugger all in teh house and I was probably in the shed! Poppy was out playing with grandchildren....they got her on her bike!

Just checked and Jim does do 74.5s, but again you need his rods!

The barrels I originally wanted to use looked good enough to hone and fit 77mms, but in the end they were on the limit without honing! Hence the 77.5! so I had the knackered barrels I got from Den Etheridge linered for the other pistons....

Chris,
what are you using for ignition? Jim

Hi Comnoz

Been using the Interspan set at 31 degrees. Motor had been running well. When I built the bike it had a boyer Micro but would never start.
Do you think its ignition? Saying that the engine was popped in for the last meeting & had not been run for a year. It only did about 20 laps over the weekend.

I was apt to blame the pistons, as I was always told not to use them. Its a Maney stage 3 head ie 5mm over inlet. Thought perhaps the pocketing was to much.
I haven't removed the valves yet (I lost heart) but wonder how bad they will be, I seem to be missing the bits of piston & a small piece of top ring.
Barrel is remarkable unscathed. Spec for the engine as I brought it said Maney/Cosworth pistons. It was built by Martin Page. Seems it night have been swapped out!
Any advice gratefully received.

Chris

Chris,
The piston looks to have suffered from detonation. 31 degrees is a lot of lead for a big valve high compression Norton motor. 28 degrees at 6000 rpm would be a safer bet.

Your cast omega piston will break and fail as it did but a forged piston would have likely overheated and failed from seizure under the same conditions. Jim

Hi

Thanks Comnoz

Looks like I need to have a look at a higher spec race fuel? I normally run Avgas / super unleaded.
Will set the bikes up at 28. Just need to find some more pistons.

Jim do you rate the gmp pistons?

All the best Chris

Chris said:

Hi

Thanks Comnoz

Looks like I need to have a look at a higher spec race fuel? I normally run Avgas / super unleaded.
Will set the bikes up at 28. Just need to find some more pistons.

Jim do you rate the gmp pistons?

All the best Chris

Click to expand...

I would not race the GPM pistons but I would race with the JCC pistons.

Or forged JE's are available in 74mm. They are high compression [extra material on the top] and weigh a little less than stock. They have cutouts for big valves. Jim

I should have added.
I would not race with the GPM pistons mainly because I have little experience with the more recent GPM pistons. Jim

FWIW, Steve Maney used Cosworth pistons up until mid 2000, when he switched to JE pistons. Steve and I collaborated on the design, mostly just a copy of his design from Cosworth, but with some small changes, one of which was lowering the top ring a bit to avoid problems with the valve relief being too close to the back of the ring groove.

Ken

If your bores are still good at 74.5 mm, you might check with Steve to see if he has any 74.5 pistons in stock. We did have them made in that size. Back then, we had them sized for .002" clearance in the nominal bore sizes, so the bores had to be honed another .003" to get the .005" clearance we recommended. That allowed us to clean up bores that were only slightly scored, without having to bore to the next oversize. We no longer do that, but if Steve has any in 74.5 on the shelf, they will be the earlier ones. If your bores will clean up at +.003" or so, you could use these pistons.

Ken

Chris said:

That's a big valve cut out? And yes "both pistons"

Chris

Click to expand...

That's an awfully thin margin between the valve cutout and ring groove. I realize it's too late for these, but could you get away with a smaller diameter/shallower valve cutout on the next piston set? The only other way I could see to increase the margin is to drop the ring pack deeper. With the generous area above the gudgeon pin, that'd be one way out. Of course, you'd need to find out if they'd be willing to custom lathe a set for you.

Comnoz said:

Chris,
The piston looks to have suffered from detonation. 31 degrees is a lot of lead for a big valve high compression Norton motor. 28 degrees at 6000 rpm would be a safer bet.
Your cast omega piston will break and fail as it did but a forged piston would have likely overheated and failed from seizure under the same conditions. Jim

Click to expand...

I agree with Jim's summation, yet I don't see other signs of detonation that I'd expect; e.g. eroded/exposed areas on the piston crown and/or head, although not all the crown is visible. Granted, if you had a sudden onset of detonation, it sure could bust the piston before any other signs surfaced. And, it's too late to take a magnifying-glass look at the plug ceramics, as I'm sure they're littered with chunks of aluminum when the pistons let go. Assuming it was detonation that caused this, I'd also suspect the condition of the rods and big-end bearing shells.
Once back together, definitely start with a conservative ignition setting, then sneak up on it until power output and/or plug readings began to show signs of entering the danger zone.
Nathan

I have one of Steve'scrank ignitions and he reckons for the compression I will run to set at 29, for a higher compression 28.

Norman told me to run 31!

And obtw, Norman has JE pistons in stock, ckeck with him for size, they look a lot like Jim's, but differ in detail, apparently he has used them for years, I think they are for standard length rods!

The first and second pictures indicate to me that you may have been pushing an abundance of oil past the rings and into the combustion chamber. This is especially apparent with the side view of the piston where the oily/tar between the rings and oily carbon between the top ring and crown.

With high performance engines, oil bypass to the combustion chamber is an effective way to bring on detonation.

Chris said:

Click to expand...

Dances with Shrapnel said:

The first and second pictures indicate to me that you may have been pushing an abundance of oil past the rings and into the combustion chamber. This is especially apparent with the side view of the piston where the oily/tar between the rings and oily carbon between the top ring and crown.

With high performance engines, oil bypass to the combustion chamber is an effective way to bring on detonation.

Click to expand...

Sorry to state the obvious, but when the top side of the piston blew out, wouldn't that let the oil into the combustion chamber? Not sure all the oil and tar tells anything in this case.

When I blew a piston on my 850, it had blown the ring groove all the way through to the oil ring. The blow by had pressurised the oil out every seem in the engine, but it never fumed like you would expect and it never blew smoke. While it was going it was tricky to diagnose and lack of experience with 850's meant I did not notice the compressions were down. I always wondered why my 750 went like a rocket in comparison. Now the 850 goes like a rocket with the new pistons fitted.
Hope this helps to explain why you may not have been using oil before your piston blew.

dennisgb said:

Dances with Shrapnel said:

The first and second pictures indicate to me that you may have been pushing an abundance of oil past the rings and into the combustion chamber. This is especially apparent with the side view of the piston where the oily/tar between the rings and oily carbon between the top ring and crown.

With high performance engines, oil bypass to the combustion chamber is an effective way to bring on detonation.

Click to expand...

Sorry to state the obvious, but when the top side of the piston blew out, wouldn't that let the oil into the combustion chamber? Not sure all the oil and tar tells anything in this case.

Click to expand...

Ah, both cylinders? See the combustion chambers. Keep in mind this is a race bike, not a street bike. Although obvious that there was some oiling post incident and maybe upon dismantle, there are some not so subtle clues (aka obvious) such as the dark baked oil between the ring grooves and the thick dark carbon in and around the top ring which is not normally associated with an over rich engine.

Dances with Shrapnel said:

Ah, both cylinders? See the combustion chambers. Keep in mind this is a race bike, not a street bike. Although obvious that there was some oiling post incident and maybe upon dismantle, there are some not so subtle clues (aka obvious) such as the dark baked oil between the ring grooves and the thick dark carbon in and around the top ring which is not normally associated with an over rich engine.

Click to expand...

You could be right, but he did say in the original post that both cylinders went...or maybe I read that wrong. The thing is this happened during a race, so if the engine was working hard and kept going for a bit, the forensics are a bit difficult. For sure it dumped oil into the combustion chamber(s).

One thing that is telling is the combustion chamber is relatively clean. If it had been blowing oil, why so clean? It would seem that the charring and tar is from the heat of the engine when it blew and the oil came through.

Hi all

Thanks

Things to take on board. It did puff smoke on the over run braking into corners occasionally, but then not again for most of the race. The carbs were set up rich, it started & ran well with good power but I always felt it was a bit rich. I have left a message for Steve ie his pistons but can only hope I will be fortunate. The barrels look great but I will forward them to whoever supplies the pistons.
Bottom end will be coming apart as there is no point not to. In for a penny.

Keep your views coming. I intend getting this engine back out there. It was a joy to ride revved to 6600 with ease but was nice to ride.

all the best Chris

ps Comnoz do you have any references for the forged je or jcc pistons? The website is for modern stuff, I will need to contact them.