B25 piston

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BasilF
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B25 piston

Post by BasilF »

IMG_20200429_162626.jpg
IMG_20200429_162633.jpg
Is this the standard piston for a 1971 B25? None of the illustrations I can find show a split skirt. There appears to be no markings for front and back. There is also a small witness mark on the crown where the sparkplug may have been touching it.
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Re: B25 piston

Post by madsundaysurvivor »

It is not easy to see if I do not have the piston in my hands. it is rare for the spark plug to rub against the piston no matter how much it expands, before would touch the valves head. Rather it seems that they have used a tool to block the crankshaft through the spark plug hole, surely to tighten the alternator or timing nut
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Re: B25 piston

Post by minetymenace »

madss wrote:it seems that they have used a tool to block the crankshaft through the spark plug hole
Good thinking batman!
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Jeff K
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Re: B25 piston

Post by Jeff K »

I am thinking one of the cheap TDC gages. The ones with the spark plug like body and the rod that goes up and down. I have seen people in the past instead of putting the bike on a stand, into 4th gear and slowing bump the rear tire to slowly move the piston up and down. They pump the kick start lever up and down and sometimes jam the rod in place.
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Re: B25 piston

Post by beat »

I have had once such a mark on a B50 piston , made by a pice of ceramic isolator falling off a spark plug.
surpricingly the engine was continuously running, evan with a plug gap of 0.1 mm only.
just a short " knock " was to feel.......

:oops:
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Re: B25 piston

Post by JB »

BasilF wrote: Wed Apr 29, 2020 4:56 pm Is this the standard piston for a 1971 B25? None of the illustrations I can find show a split skirt. There appears to be no markings for front and back. There is also a small witness mark on the crown where the sparkplug may have been touching it.
Genuine BSA pistons were a solid skirt type and would have BSA clearly cast on them, Hepolite pistons (who were a Triumph OE supplier), were, as far as I know, never factory fitted to B25's and were of a split skirt design similar to yours.
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Re: B25 piston

Post by BasilF »

Thanks for the replies. The issue at the moment is if I decide to fit new rings then where do I get them from. Also the bore seems to be 10 thou over size while standard BSAs are 20, 40 and 60.

Searching through the web it seems there are several vendors offering Hepolite pistons for B25s and the rings seem to be compatible with the BSA pistons The Hepolite pistons on sale have the oversize stamped on the crown so I'll do some more scraping and polishing tomorrow.
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Re: B25 piston

Post by FranktheWelder »

Well this is all very interesting. I have the same piston you do with no forward mark at .020" oversize. My bore was fine and didn't get measured and ordered standard BSA rings not aware mine was not stock. I received my rings today but didn't compare or test fit to barrel yet.
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Re: B25 piston

Post by Jeff K »

Check the inside of the piston, under the wrist pin for marks and numbers. Sometimes they are there. Long time ago rings and a few pitons were available in +.010" , +.030" etc. Here are some photos of some B25 pistons. The "solid" ones all have BSA on the outside by the wrist pin. The "slotted" one have AE on the inside. These were from a box of used piston. Four of them were +.040" that tells me the over bores were common with the B25 back in the day. The solid "BSA" pistons have a thinner wall then the slotted AE pistons.
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Re: B25 piston

Post by kommando »

AE stands for Associated Engineering, a group that owned Hepworth & Gandage who made Hepolite pistons.
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Re: B25 piston

Post by koncretekid »

If your piston and bore are in good shape, and the clearance is OK, you could probably get away using +.020" rings on the +.010" piston, making sure the ring gap is sufficient. I did that on a race bike once (CB350 with K*wasak* pistons) and never had a problem.
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BasilF
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Re: B25 piston

Post by BasilF »

IMG_20200430_151747.jpg
16945 is the part number, 413 is the material, don't know what 1B74 or G1 are. My cheap Vernier calipers won't give an accurate diameter of the bore or piston (anything from 67 to 67.5.)
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Re: B25 piston

Post by beat »

piston and cat almost similar : massiv :!:
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Re: B25 piston

Post by minetymenace »

But the piston (unlike the cat) don't need Gaussian compensation >44
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Re: B25 piston

Post by crankbuster »

Here's three common ones:
pistons.jpg
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Re: B25 piston

Post by Jeff K »

Great chart of the piston designs! It clearly shows the BSA has a thicker dome area and thinner walls then the AE piston. Back when I used to do a lot to 250 work. I would always order the rod bearing set for the Triumph 500 twin. They are the same as the BSA 250 , but for a few dollars more you would get two sets. I also seem to recall that the ones used for Triumph where of a better material. The same for the rings, I would order rings sets for the BSA/Triumph 750 Triple, again the same as for the BSA 250 single and for a few dollars more you got three sets. I wish that there was a list out there that would have all of the "common" parts that were the same between models and companies.
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