The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 1946-1950 => Topic started by: Craig on 14.07. 2016 09:35
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Hello
New to this forum and thanks for the add. The specific Longstroke section is a great idea. My bike is 49 Longstroke Plunger.
I am currently doing a total “top end” rebuild and seeking some advice from the group.
So far have done. New GPM +40 pistons (63mm) plus re-bore. New valves and valve guides and springs. The head has also been skimmed down 0.006”. New solid copper gasket which will go on.
The new pistons are about 1.55mm longer (from top of crown to bottom of skirt) longer than the original pistons. The deck height of the new pistons is same as old ones.
When I stripped it all down I found a compression plate on the barrel. Apart from obvious reason for using a compression plate I am worried that PO did this for some strange reason. (have evidence of questionable mechanics on bike from the past). I am inclined to leave this compression plate off as would prefer the “standard / higher” compression.
Given the longer pistons and the skimmed down head, am concerned about the overall clearance of pistons to head etc when fully assembled and compression plate removed.
I will do a test fit of everything soon before final assembly. But I wonder if any on the group have any advice for me on possible issues, things to check.
Just want to be as sure as possible that all works well and will be no issues. I am new to this fantastic bike but handy enough with the tools.
Cheers
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g'Day Buddy, *welcome*; 'pinkpale' is something of a mouthful- if you don't like 'pinko' or similar let me know ? (It's bound to happen anyway)
When you say; The deck height of the new pistons is same as old ones.
, I presume that's from the pin ^up^, so that means the skirt is past knee length? *smile*.
Won't be long before the others hit you up for more detail ie; where you at, and other stuff like piccies
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Hey Dutch
Craig is the name - but you can call me Susie if it makes you feel better i am not bothered.
Deck height is from the center of the gudgeon / wrist pin to top of the piston (the deck).
I am based in Hong Kong. The only A7 (and long stroke) here.
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G'day Susie (I couldn't resist *smile*).
If it's only 6 thou that's been taken off the head it would be OK. But we don't know how much the PO had skimmed off it or the barrels. The only way to be sure is if one of our Longstrokers could measure the thickness of a std head and barrels. You could test it by laying a rolled up 1/4" thick piece of Plasticine and lay it front to rear over the piston, then assemble the head and wind the motor over 4 revolutions. Disassemble and check the thickness of the Plasticine. There should be at least 60 thou (1.5mm) clearance between the piston and valves.
Cheers
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Cheers Muskrat - brilliant advice.
will give it a go int he coming week and report back for interest sakes
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Muskrat - thanks again for the advice. Worked a treat. Hardest part was finding the source of play dough with not having any sprogs.
Did the test fit and found approx 1.68mm clearance.
Top end all back together with out compression plate and with new pistons and re-bore. Compression is noticeably much much better just from few kicks. Old girls is going to get up and go very well.