Which pistons are available for my ariel

[Greg.Pawson]

The engine has standard VH cam and followers, 7:1 com and it goes like stink....!
The vibration it had with the old piston has gone.
Two pics of the old piston attached

I have the ignition timing and carburation spot on, and have no worries about piston seizure it feels free and I enjoy doing big throttle roll on's

This piston complete weight is 510g

[Greg.Pawson]

Very low hour 1956 HS MK 1 piston, @ .020" over.
The complete weight is 480 g

[cmfalco]

To add something to this thread, I just removed my 1928 Model C's piston and found it to be a Hepolite (Heplex) 10793 that a pdf on the Draganfly site says is a 6.3:1 piston used in 1935-58 VG/VF/VH. Since the connecting rod has a small end for a 1" gudgeon pin it has been sleeved with bronze for the later 13/16" pin. The total weight of the piston, pin, and rings is 466.5 grams.

I haven't been able to find the factory balance factor anywhere, but to the extent I assume this Hepolite weighs about what the original Hepolite for that engine did, and if there are no signs of relatively fresh balancing holes in the flywheels when I get further into the engine, I can use its weight to get a good estimate of the balance factor to use with whatever piston I'm able to locate to replace the well-used one I just removed.

Although not many miles were put on it between being rebuilt by some unknown person and me getting it three months ago, it was running fine with this piston. I don't remember it having been noted earlier in this thread, but for other owners of OHV Black Ariels this shows the pin-to-crown dimension of a later piston is appropriate for substitution.

Unfortunately for me, the piston is 0.040-over and Draganfly only lists 0.060-over aftermarket pistons in 7.5:1. I've not run an iron-head engine before but that seems to me to be a bit on the high side. Perhaps there is enough material in the crown for me to machine some away. What experiences have other people had with a CR this high in an iron head engine?

[Simon.Gardiner]

7:5 to one was the factory 'high compression' option for the iron-head VH, there will be no problems with that ratio. I (and many others I suspect) have run many miles with the Bob Metson AOMCC 8:1 piston in iron head 500s, it goes nicely, pity there are no more available.
The Drags web-site shows JP pistons which are usually heavy and have a mixed reputation for reliability (plenty of references to those in the forum). AOMCC spares (Pete Kemp) commissioned some 7.5:1 from Omega which are lighter. If you go back to page 7 in this thread there is a pic of the Omega and also a 10793.
I've been weighing a few old VH pistons that I've accumulated and they're all over the place, the 7.5:1 is heavier than the 6.2:1 (as might be expected) for which it was a factory 'fit and forget' replacement, and I've found an early example of the 7.5:1 (three compression rings) that is between the two, so even different types of nominally same piston had different weights. When I've got a bit more time I'll try and put the info up here along with some photos.

HTH

SG

[cmfalco]

Simon,

Thanks for your post. I've emailed Peter to ask if he still has any of those Omegas.

Since no one seems to know the balance factor the factory used, it would be very useful to know the weights of as many old pistons as possible. I hope you find the time to assemble the information from the pistons you have.

[cmfalco]

Update: Peter responded almost immediately to my email, has the +60 I need, so a 7.5:1 Omega is now on order. Simon, thanks again for pointing this out. I had read this entire thread but somehow missed the fact that these pistons might still be available.

[david.anderson]

Iron engine Ariel singles are a bit scanty with the finning and the black Ariels possibly even more so. With the milder camshaft fitted to the 28 than was fitted to later singles, the inlet valve closes earlier on the compression stroke. That effectively raises the dynamic compression ratio which increases the likelihood of pre ignition. The 7.5 (Static compression) piston may be at the limit for a 28. Perhaps someone who has run a 7.5 piston in a black Ariel could comment.
I did mention in a previous post the balance factor that I run in a VH being 66.6%.
http://forum.arielownersmcc.com/viewt ... =20#p42886
Ariel actually quoted a balance factor of 65% for the later line of singles, however I found at higher speeds the 66.6% balance factor is much smoother from 60mph.
David

[cmfalco]

The 7.5 (Static compression) piston may be at the limit for a 28. Perhaps someone who has run a 7.5 piston in a black Ariel could comment.

David,

Believe me, this is one of several things that keeps me awake at night. Horsepower isn't an issue for how I will use this machine. Reliability is. However, after scouring the known world (plus Australia) for 6:1 pistons none were to be found. I'll only know after the Omega piston arrives if there's enough material in the crown for me to safely shave off enough to significantly reduce it from 7.5:1 but, if there is, that is part of my plan.

Before Simon Gardiner pointed out the existence of the Omega pistons I already had ordered an aftermarket 7.5:1 from Draganfly. No matter what it turns out to be (probably JP), when all the pistons are in hand I plan to do a careful comparison of the thermal expansion of it, the Omega, and the Hepolite. I've read speculation that the JP has "much larger" thermal expansion than stock, but have seen no data on whether or not this actually is the case.

On the subject of pistons, I machined a torque plate and measured the effect it has on the shape of the bore. Anyone interested in knowing the result will find it posted in the next day or two in the rebuilding thread at:

http://www.britbike.com/forums/ubbthrea ... Post691207

Perhaps someone who has run a 7.5 piston in a black Ariel could comment.

Yes, please do!

I did mention in a previous post the balance factor that I run in a VH being 66.6%.

I did note the balance factor you used, but you wrote "In a 49VH rigid frame I am using a 2/3 or 66.6% balance factor and the bike is vibration free. I would expect the same result with the 28." Although the frames are similar, the details of the engine mounting, cycle parts, and telescopic forks of yours all have significant effects on resonances/vibration. Although all singles from all manufacturers basically use similar balance factors the "optimum" for each is different because of these factors.

[ian.scott]

Why couldn't you put a compression plate under the barrel to bring the compression down ?

Ian

[cmfalco]

Why couldn't you put a compression plate under the barrel to bring the compression down ?

A compression plate alters the rocker geometry resulting in additional sideways thrust on the valve stems unless compensated for by longer pushrods. Already wear of the exposed semi-unlubricated valves and valve guides are of concern so I don't want to do anything to make the situation worse.