E: Engine CR and ignition timing on a B series single

[Simon Dinsdale]

Years ago I measured the compression ratio of my C Rapide which was fitted with the clubs 7.3:1 Omega pistons, and they came out at 7.2:1 with a volume of 80.5 cc.
I have never measured an 8:1 piston.

Don't forget its not just the cylinder head volume that counts as the shape of the top of the piston also affects it. My measurement was with the engine on the bench and supported so the spark plug hole was pointing directly up and the piston was at TDC with grease around it to seal to the bore. I measured the amount of oil it took to fill the combustion chamber to the bottom of the plug threads. I was more curious if both cylinders were the same rather than the actual figure.

Simon

 

[craig]

Corey, All your details sound fine to me. My Comets have done tens of thousands of miles with a TTO meter monitoring plug base temperature. I highly recommend the TTO meters and they are easy to install and self powered with built in battery. They last for 4-5 years, cost $50 or less. I cut the spark plug metal gasket off and just use the copper ring sensor as plug gasket.

As you climb hills the temp rises and temp drops as you descend. You can run various throttle setting, drag race, change plug heat ranges, put on a sidecar.

I run two TTOs on a twin.
I run oil temp on a Comet, at the bottom UFM banjo, just for fun. I think it got to 135F on the hottest day run.

 

[davidd]

Corey,

I don't see anything that jumps out. It is a conservative spec. I would break it in correctly so the rings bed and check the plug for heat. It is difficult to check new clean parts, but give it your best. All should be well.

The following does not necessarily involve Corey. I know that Omega pistons have been used in Vincents for years and I did not know they had a defect. Maurice Candy runs them presently in his championship Manx and has been doing so for a decade, at least.

I will say the the Carillo rods I run have slightly more clearance in the small end than the street rods. Carleton came up with this spec due to the excessive bluing on his race bike pin. This seems to happen with the racing singles. It never seemed to prove fatal, it was just an indicator. We were both running 0.020" plus clearance on the big end sides. That 0.020" is the total for both sides. It is important for the big end to get rid of the oil so the rollers don't skid and it is also important to have the big end supply oil to the underside of the piston as best it can.

We also used phosphor bronze side washers like the Manx. The hardened Vincent washers would sometimes spin under side loading. This seemed more common for the Alpha bearings than others. Spinning washers would overheat easily raising the temperature of the big end and the oil.

My memory of CCing the Vincent heads is that you need the plexiglass cut in a circle to fit the counterbore in the head. Otherwise, you have to calculate the volume of the counterbore and subtract it from the total volume. As Simon mentions, you also need to know the volume of the top of the piston that intrudes past the counterbore. The piston needs to be measured at TDC as it comes out of the liner (or fails to come out of the liner.)

When measuring the piston top pick a number that is easy to work with, lik 1". Calculate the volume of the cylinder 1" down and that is the total volume. Then put the piston in the cylinder with the top 1" down, fill to the top. You have to use the plexiglass our you will have the meniscus included in the measurement. The difference in the two volumes should tell you what to subtract from the total combustion chamber volume.

David

 

[timetraveller]

Using a circle of plexiglass is exactly what I did except that there was a very slight cut out on one edge to allow the oil to be inserted from the burette. Nothing magical or difficult here to understand. If my measurement of 103.5 cc is correct then the compression ratio, with a flat topped piston, would be 5.8:1, assuming that the swept volume is 500 cc.(500 + 103.5)/103.5 = 5.8 . The dome on top of the piston just removes some of the volume from the 103.5 to give a higher CR.

 

[CoreyL]

You should have a volume of about 81cc in the head for a 7:1 which is a lot for a claimed 8:1 piston which should be about 15cc less
(I always use play dough when engine is apart a lot less mess and leakage) anyway I concur with the others given your ratio is correct everything sounds about right I think the problem is not in the head but is somewhere not addressed by your rebuild more examination along the lines suggested above is required

Iy

Years ago I measured the compression ratio of my C Rapide which was fitted with the clubs 7.3:1 Omega pistons, and they came out at 7.2:1 with a volume of 80.5 cc.
I have never measured an 8:1 piston.

Don't forget its not just the cylinder head volume that counts as the shape of the top of the piston also affects it. My measurement was with the engine on the bench and supported so the spark plug hole was pointing directly up and the piston was at TDC with grease around it to seal to the bore. I measured the amount of oil it took to fill the combustion chamber to the bottom of the plug threads. I was more curious if both cylinders were the same rather than the actual figure.

Simon

I measured my combustion chamber at 83.4 ml. The liner/piston is 0.020" oversize so the bore should be 84.5 mm (not including the 0.005" of clearance).

 

[CoreyL]

35 degrees does not sound like too much advance, you should easily be able to hear if the engine was pinking, detonating if this was an issue. How much tension did you use when pulling the head down........ ?........What crank assembly are you running, this can be an issue in part due to lack of oil getting to the underside of the piston, this usually shows if the piston pin is showing signs of turning blue, and the little end of the rod too.

I used 30-32 ft-lb to torque the head. I'm ignorant regarding the type of crank assembly I have. There was evidence of burnt oil under the piston crown and also some discoloration of the wrist pin.

 

[vibrac]

I think I will stick to my method

https://www.vincentownersclub.co.uk/threads/compression-ratios-in-the-toy-shop.8213/#post-71284

 

[CoreyL]

Corey, All your details sound fine to me. My Comets have done tens of thousands of miles with a TTO meter monitoring plug base temperature. I highly recommend the TTO meters and they are easy to install and self powered with built in battery. They last for 4-5 years, cost $50 or less. I cut the spark plug metal gasket off and just use the copper ring sensor as plug gasket.

As you climb hills the temp rises and temp drops as you descend. You can run various throttle setting, drag race, change plug heat ranges, put on a sidecar.

I run two TTOs on a twin.
I run oil temp on a Comet, at the bottom UFM banjo, just for fun. I think it got to 135F on the hottest day run.




View attachment 24594View attachment 24595

Is there a part number for that gauge? What temp would set off an alarm in your head if you saw it on the gauge? How hot is too hot?

 

[Bill Thomas]

Check the top surface of the crank case, I have seen a crack where the cam spindle goes, which can tip the barrel. Cheers Bill.

 

[craig]

Is there a part number for that gauge? What temp would set off an alarm in your head if you saw it on the gauge? How hot is too hot?

I have bought all TTO from shopmxwest using eBay and free shipping.
https://www.ebay.com/itm/Trail-Tech...h=item5449c45d0e:g:RkIAAOSwhQhYxYL1:rk:1:pf:0

Just type at eBay - tto 14mm

My Comet 8/1 piston, Mk1 28mm, on a hot day , 95F ambient, 10 mile steady pace 65 mph , 200 lb load, flat ground, TTO might read as high as 340F.
Climb a 3 mile hill at 65mph might be little higher.

My Comet TTO typically reads 290F - 320F. speed comes down, temp comes down. I typically ride back roads at 50mph or less, but pretty aggressively.

You have to Z bend the copper rings to fit Vincent head form, but easy to figure out. I also ream out the hole, Dremel 1/4 sand drum, to make it easier to take on off, as when it crushes it gets smaller and wont come off the spark plug nicely.

I would double stick tape it to a simple aluminum strap to one side, one hole , mount to the upper Smiths speedo stud.
Cheers
Craig