ET: Engine (Twin) TAMING A BIG BORE

vibrac

vibrac

Well Known and Active Forum User
VOC Member
After its hard work over the last few years and recently in the IOM I need to refresh my road Comet now my thoughts are to use one of the big bore barrels I had on the racer (before it dawned on me that we got more points for coming second with a 500 than first with an 600 in our class ;)) trouble is, in deference to the fact there is still no electric start for a Comet I want a compression ratio of around 8:1 or lower and all the 90 bore pistons I have are like small models of the Alps. I know I can add packers under the barrel but that soon exposes bore above piston and not a way that is my first choice. Has anyone used a more flat top piston off something else?
 
stu spalding

stu spalding

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vibrac said:
After its hard work over the last few years and recently in the IOM I need to refresh my road Comet now my thoughts are to use one of the big bore barrels I had on the racer (before it dawned on me that we got more points for coming second with a 500 than first with an 600 in our class ;)) trouble is, in deference to the fact there is still no electric start for a Comet I want a compression ratio of around 8:1 or lower and all the 90 bore pistons I have are like small models of the Alps. I know I can add packers under the barrel but that soon exposes bore above piston and not a way that is my first choice. Has anyone used a more flat top piston off something else?
Click to expand...
Mac Reed put an electric starter on a Comet. That's all I know. Cheers, Stu.
 
Marcus Bowden

Marcus Bowden

VOC Hon. Overseas Representative
VOC Member
Paul Hamon is going back onto the case for a Comet starter I found out 10 days ago when there picking up two Alton generators, new castings have been done of the back primary case and machined for the conversion kit.
ALTON <alton-france@orange.fr>.
Ask him your self, he might hurry up if people keep asking after them, he knows I want one ! some thing to do with doggy knees.
bananaman
 
vibrac

vibrac

Well Known and Active Forum User
VOC Member
Comet Rider said:
Tim,
What pistons do you have?
The Omega with my 90 bore had lots of crown thickness so it could be machined to give about 8.2:1 CR

Neil
Click to expand...
My 90 bore piston has a cartouche shaped depression on the centre underside of the crown so no shaving allowed
however in spite of what I said about packers it occurs to me that on a 90mm bore just 1mm of packing gives me 6cc of compression space (about 1cc per 6thou) so not far till I reach non straining kick ratios and I can chamfer that bore edge near the inlet to off set the power loss better I suppose than using some 90 bore car piston
 
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