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#371542 - 05/04/11 2:30 pm piston cylinder clearance, A65
oleandreas Offline
BritBike Forum member

Registered: 05/31/06
Posts: 198
Loc: Tynset Norway

Bsa A65, 1969. old 750 kit. Devimead.

Anyone have any opinion on a piston - cylinder clearance on 0,25 mm = 0,0984 inches.

I have recently got a Devimead cylinder, seems to be almost newbored, bore 80,05mm. No edge on the top inside the cylinder from wear, the pistons that came with measures 79,80mm, 90 degrees on the writspin, down on the piston. (79,25 mm on the top).

Is it worth trying with this clearance ? I know Normally its reccomended around 0,11 mm. I have cast iron cylinder and as far as I know the pistons are forged.

Alternatively Im checking with Ed V if he migth have some suitable and lighter pistons to sell me. These pistons I have here with the Devimeade cylinder weights 430 g with writspins, the Ed V pistons (B44, JE) I have for another project in bore 79,5mm weights 385g included wristpin. I guess lighet is always better.. IM using stock A65 engine, stock crank, ported head and bigger valves

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#371543 - 05/04/11 2:43 pm Re: piston cylinder clearance, A65 [Re: oleandreas]
wak Offline
BritBike Forum member

Registered: 03/29/11
Posts: 798
Loc: derby england
.010" sounds a bit big .005" is the norm and as worked well for me.
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#371546 - 05/04/11 2:57 pm Re: piston cylinder clearance, A65 [Re: oleandreas]
kommando Offline


Registered: 12/31/04
Posts: 5821
Loc: Scotland
Clearance will depend on the pistons you fit, the material they are made of if a big factor plus if they are cast or forged.

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#371547 - 05/04/11 2:57 pm Re: piston cylinder clearance, A65 [Re: oleandreas]
Alex Offline
BritBike Forum member

Registered: 09/05/02
Posts: 7574
Loc: NorCal
^^^What he said^^^^
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A smattering:
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#371555 - 05/04/11 3:52 pm Re: piston cylinder clearance, A65 [Re: Alex]
oleandreas Offline
BritBike Forum member

Registered: 05/31/06
Posts: 198
Loc: Tynset Norway
If I understand you right, the clearance seems to big.

I have forged pistons, and cast iron sylinder. guess this set is from the 1970 or 1980s...

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#371556 - 05/04/11 3:58 pm Re: piston cylinder clearance, A65 [Re: oleandreas]
kommando Offline


Registered: 12/31/04
Posts: 5821
Loc: Scotland
According to this site

http://www.glmmarine.com/castvsforged.html

Pistons: Cast vs. Forged

GLM offers an extensive line of pistons for all major brands and models of outboard engines. GLM pistons, like OEM pistons, are cast as opposed to forged.

We have all heard the advantages of forged pistons. However, when you consider the disadvantages of forged pistons it becomes very clear why GLM cast pistons are the ultimate pistons for your rebuilding and repair projects.

The major disadvantages to forged pistons are actually a result of the forging process itself. Forging results in a piston that is considerably heavier, than cast pistons, and is limited in the aluminum alloys that can be used to produce the piston. Additionally, the forging process also limits the design configuration of the piston itself.

The forging process and its limited choice of aluminum alloys result in a dramatically higher expansion rate for the forged piston. This means that the set up characteristics are very different from the original engine manufacturers' and that break-in and warm up periods are crucial for the forged piston.

An example of the dramatic expansion of forged pistons is the piston skirt clearances in the cylinder. A typical forged piston has a skirt clearance of approximately .0007 to .0009. A cast piston has a skirt clearance of .005 to .007. The forged piston has 10 times more slop in the cylinder. This results in less ring stability against the cylinderical wall, more piston noise and extra blowby.

In all fairness, after the forged piston has reached operating temperature, its dramatic expansion makes up for these extra clearances. However, this should remind us of the typical customer that a dealer services. Can we expect the casual weekend boater to strictly observe the extended break in period and the critical warm up procedures required for a forged piston? Let your own experience answer this question.

Most forged pistons are quality products, but they are better suited to racing and professional applications.


Note only the manufacturer of the orginal pistons knows the recommended clearance, asking a forum with incomplete info (you don't say which brand pistons they are ) is asking for trouble.

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#371561 - 05/04/11 4:33 pm Re: piston cylinder clearance, A65 [Re: oleandreas]
Alex Offline
BritBike Forum member

Registered: 09/05/02
Posts: 7574
Loc: NorCal
When in doubt, 1.5 part/thousand clearance has not failed me once.
_________________________
A smattering:
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'67 Royal Star
'71 Rickman Metisse
'40 Silver Star
sixtyseventy Lightboltrocket road racer...and many more.

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#372055 - 05/08/11 2:23 am Re: piston cylinder clearance, A65 [Re: oleandreas]
jaycee Offline
BritBike Forum member

Registered: 09/17/08
Posts: 517
Loc: angeles city, rp
i read the post three times,as the link.im not the smartest fella in the world i admit,but i must respectfully submit that .0007-.0009''is significantly LESS than .005-009"

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#372104 - 05/08/11 1:16 pm Re: piston cylinder clearance, A65 [Re: oleandreas]
Mr Mike Offline
BritBike Forum member

Registered: 08/06/01
Posts: 3610
Loc: Cape Carteret, NC
I think what Alex is saying is a 3" diameter piston gets 3 x1.5/1000 equals .0045" clearance or .114mm. Alex, correct me if I am wrong. Ed V has bored a bunch of cylinders for me and not a smoker of slapper in the bunch. Send him your cylinder and he'll provide the piston and correctly fit them. If you wnat to use your piston send it also and he'll tell you what to do.

Mr Mike

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