Author Topic: 10.5:1 pistons  (Read 1490 times)

Offline nimbus3d

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10.5:1 pistons
« on: 21.04. 2011 00:21 »
Hi everyone. I?m new to the forum having only found it recently. Lots of great information and insights on the A10 that I could have used a few years ago. 

I?m in NV, USA and have several BSAs including a 1961 A10 that is posing as an RGS- but doesn?t quite make it! I bought it 9 years ago from an owner who had it ?professionally? built by a local restorer, who did some strange things to it, like using 10.5:1 pistons (more on that later), piston rings with 52, 42 and 28 thou end gaps,- the bike would smoke like a WW2 battleship trying to hide, which is why the PO sold it,  a magneto that would not fire when hot, worn out gearbox bearings and other travesties.

My Question: After the timing side bush failed last fall, I took it all apart for a replacement and discovered that the Hepolite pistons were 10.5:1. Since I got this bike, I have ran it either on 50/50 91 octane pump gas and 100LL aviation gas, or a mix of pump and 110 racing gas. Any less than that and it knocks like crazy under load. Has anyone had good experiences with 10.5:1 pistons? Is there a change to any other engine part that should be done to work with them? Should the timing be reset from the 11/32 before TDC? Should I buy lower comp pistons? I rather not spend the $s but if these are not a good idea I?d change them the next time its apart.

Thanks in advance for your comments.  Sam.

Online trevinoz

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Re: 10.5:1 pistons
« Reply #1 on: 21.04. 2011 03:20 »
Sam,
              I tried 10.5:1 pistons in my bike over 40 years ago.
The damned thing would not run on pump petrol, pinged it's heart out, so I had to use high octane racing fuel at a 50% premium.
Then they caused the barrell to break off around the flange!
Read up previous posts re advance. 5/16" seems to be the ideal.
I run 8.25:1 in rockets and 7.25:1 in Flashes.
  Trev.

Offline muskrat

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Re: 10.5:1 pistons
« Reply #2 on: 21.04. 2011 10:20 »
G'day Sam, welcome to the forum.
 I'm running 10.5/1 MC offset pin racing pistons in the cafe. Using 98 oct boosted to 105 and it still pings at 5/16" BTDC so I don't let it drop below 3 grand  *eek*
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline A10Boy

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Re: 10.5:1 pistons
« Reply #3 on: 21.04. 2011 11:12 »
I'm with Trev. If you want a race bike 10.5 :1s are fine, if you want a bike that is rideable on the road 8.25:1 in rockets and 7.25:1 in Flashes is ideal.


Regards

Andy

1960 A10 - Black Golden Flash
Plus
1974 Kawasaki Z1a
Yam XJR 1300

Offline muskrat

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Re: 10.5:1 pistons
« Reply #4 on: 21.04. 2011 13:32 »
I agree as well. The cafe is just for a bit of fun, one hundred miles tops. Any more than that I jump on the 7.5/1 A7.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline beezalex

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Re: 10.5:1 pistons
« Reply #5 on: 21.04. 2011 14:11 »
Ditch the 10.5:1 pistons.  The stock crown shape on those things was terrible.  I wouldn't use them even in a race bike.  If you want more performance, have somebody who knows what they're doing flow the head.  This is a much better way to get more go.
Alex

Too many BSA's


Offline nimbus3d

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Re: 10.5:1 pistons
« Reply #6 on: 21.04. 2011 16:17 »
Thanks guys. It seems the concensus is to change the pistons. This bike started out as a Golden Flash and was probably rebuilt by the PO with performance in mind. Didn't work out!

I had the head/valves reworked by SRM, so its good. I doubt that the cam is changed from original, but didn't think to look for a number on it while it was apart. Should have!  Crank was reground and new main bearings and big ends installed a few weeks ago so they should be OK for the 8.25 piston, or should I go lower?

Does anyone familiar with Hepolite pistons know if they marked the crowns with oversize numbers? The existing ones are unmarked and I suppose they are standard size, but will measure the barrels before buying new pistons.

I rode the bike yesterday for the first time since the timing side bush went and it sounds good and runs properly, although I'll go easy on the new bearings for a few miles.

Thanks for all the help!  Sam.

Offline dpaddock

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Re: 10.5:1 pistons
« Reply #7 on: 23.04. 2011 21:49 »
"Measure twice, cut once" - this old adage is the byword when sorting out pistons and other ancient stuff.

You say "this bike started out as a Golden Flash..." The Flash has an iron head which is certainly not adequate for 10.5 pistons on gasoline; a hotter cam makes the situation all the more impossible. Go with Trev and A10Boy suggestions for roadwork even if you have an alloy head fitted.
 
If memory serves, Hepolite stamped their standard piston crowns "STD"

David
David
'57 Spitfire


Offline nimbus3d

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Re: 10.5:1 pistons
« Reply #8 on: 24.04. 2011 00:00 »
Thanks David, I will go with the lower comp pistons and measure carefully. This engine is 1961 and has an alloy head which may be off something else. Maybe they were made with alloy heads in 1961.       Sam.

Offline Smithgd

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Re: 10.5:1 pistons
« Reply #9 on: 26.04. 2011 16:18 »
A suggestion: I am running 9:1 with the alloy head, 357 Spitfire cam, and it runs just fine. I would suggest when you change pistons, to go with Wisco, JE, Vanola or something else modern. The Hepolites are really heavy by todays standards.  Remember that the only reason to increase comp ratio is to compensate the loss of cranking pressure, due to camshaft change. Comp ratio as a raw number does not mean a thing. It is the reading on the compression gage that is important.

Offline nimbus3d

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Re: 10.5:1 pistons
« Reply #10 on: 28.04. 2011 03:31 »
HI GD, thanks for the suggestion and the insights. I'm going to wait until the engine next needs a tear down and make the piston change. At that time it will be the 9 or 8.25:1 that I use depending on what is avaiable.   Sam.