Morning All,
Does anyone have a 7.5:1 A65 piston that they could weigh please? I want to reduce the weight of some 9:1s to maintain the balance of the engine but I forgot to weigh the 7.5:1s before binning them.
Dave.
They vary a lot depending on make, GPM seem to be lighter than hepolite. My Ed V (JE) pistons are a lot lighter than the GPM 7.5:1 were.
From memory the GPM was about 420g complete
The B44 je pistons are 396g (for comparison)
My Ed V JE A65 pistons were 380g ish (but I did have them lightened also)
Hi,
I am not sure what make they were but they had been in the bike since at least 1995. If I get a few more weights I will see what the average is and perhaps go for that.
Dave.
I dug out some pistons and weighed them.
What looked like 10:1s weighed 467 and 472 with pins
" 9:1s " 416 with pin and rings, 341 with no pin and the pin 74 so 415
" 9:1s " 297 and 293 without pins or rings BSA marked with part number and 1mm o/s so probably 371 with pin plus a bit for rings making no more than 400gms.
Unless someone can weigh some 7.5:1s to help me I will use 400gms as an average to compare the ones I am using. That is tomorrows job.
Dave.
The pistons weighed in at 426gms and 429gms with the gudgeon pins and the pistons alone being 359 and 361. Not too different but if I use the 400gms as my target weight I think it will be difficult to remove 26grms from one piston and 29gms from the other.
I suppose I will start by getting 3grms off the heavier piston to get the balance between them spot on. Then I will be able to see if I can reduce them any further.
Just a panic thought - what if they need to be heavier???? Sticking the filings back on would be a challenge.
Dave.
Well they are now both the same weight and I managed to get a few grams off so they are now 357 grams each. The 'sharp' angles on the piston crown were rounded and smoothed. This removed a bit of weight and in theory reduces the likelihood of detonation. The heavier piston skirt was then reduced by a few millimetres on a belt sander until the same weight as the other. Final polish to remove any burrs. They are, as I type this, in the dishwasher remove any remnants of Solvol etc.
I think that to take 26 to 29 grams off could possibly introduce weak spots so I will just have to see if the little bit I took off makes a difference and put up with it if not.
At least the bike has had a de-coke.
Dave.
I'm not sure if I have any photos of where mine were lightened but they were taken out under the crown, but there's quite a bit of meat in that area. The A70 super high compression of the same pistons I had custom made would have even more to go at, I can only imagine that with these anything above the area which is theoretically in the squish area (I say theoretically as if you looked at a triumph Bonneville I think piston there's a defined squish) is a solid piece... different makers may vary.
The radius'ed edges will make a huge difference. Until recently there was no aperture within my outrigger bearing plate for strobe timing so I always set the bike to where it pulled well. I recent made a less than cosmetically pleasing job making making such window for strobe timing. I set the Timing disc and dial gauge up to see where it was actually Firing at from a static prospective. If you aligned the dot on the rotor up with the hole in the stator plate it was at 48°. So it would be somewhere around there +/- probably 4°. I used to use the other tuning methods for determining spark advance (looking at the lighter spot on the spark plug ground strap being one, the lighter part should be on the apex of the strap) none of these methods ever told me anything was wrong and I never had any mechanical problems from it either. Even under load with over 10:1 compression it never pinged once.
Only photo I can find is a comparison of the 2 pistons. The A70 is shimmed up on paper as the pin to deck height is much closer
When the dishwasher has finished I will take some photos, probably tomorrow now.
There does not seem to be much meat under the crown but it is difficult to determine how thick they are. If I stand them on their head and put a straight edge across I should be able to measure internally and externally. If there is enough material I can just reduce the crown on the outside and lower the compression a bit at the same time.
Dave.
Good morning,
So pistons very clean but they went in very shiny and came out quite dull. Still very smooth so usable after a wipe with WD40. They still need a bit more fettling.
I have measured, as best I can, the thickness of the crown and they both come out at 8mm with the 'corners' being about the same. As a reference I measured the thickness of a flat topped A10 piston and that was 7mm so a 1mm reduction should be OK shouldn't it?
A dilemma. Run with them as they are or try to reduce the weight some more. Ideally, I suppose it should come out from under the crown but I have no means of doing this so what about reducing the crown from the top. This will also reduce the compression a bit which I am happy with.
Any votes for or against?
Dave.
If you could mount them in a lathe, you could shape the inside of the bottom skirt to a pointed edge, that would loose a fair few grams and help get a bit more oil under the crown. But if they weigh the same I wouldn't worry too much. Check the pins are in balance With each other etc etc And not just total weight. If it was me I'd fit those as they stand, the only rocking couple would be the crank, but I think you would find even this would be hardly noticeable and seem fairly smooth at most revs.
I think it's hard to judge the crown height of the A10 piston to the A65. With the bore and stroke being different, different crown heights, combustion chamber shape etc. With this I'd try what you have, you could also fit a plate under the barrel or double the gasket up.
Hi Allan,
I will probably run them as they are. The measurement of the A10 piston was the thickness of the crown only to compare with the A65.
Sitting the pistons upside down on the bench I lay a straightedge across the skirt and measured with a vernier to the bench and then to the inside of the crown. The A10 being 7mm and the A65 being 8mm
The 7:1 compression probably requiring a bit less thickness. The surface area of the 9:1 A65 piston is quite significant compared to a 7.5:1 one and if it is 1mm thicker that is quite a lot material.
The pins are exactly the same weight so I should have it as balanced as I am going to get without substantial work on the pistons.
I don't have a lathe and I would get bored doing it with a file so the oil supply will have to do. In fact when I took the barrels off the was plenty of oil under the pitons.
Any more balancing will have to wait for a rebore and bottom end overhaul.
Dave.