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Source for Hepolite 7892-1A Pistons?
#795043 01/07/20 1:29 am
Joined: Nov 2018
Posts: 7
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I was riding my 1970 Thunderbolt last August when it started to run poorly, then had a major loss of power. I coasted it onto a side road where we stopped and called for assistance. My son was following me on his Commando and said there was a spectacular plume of smoke coming from the left exhaust. While waiting for the roadside assistance, we pulled the left plug to find a nickel sized hole in the top of the piston.

We disassembled it about two weeks ago and other than the piston (and some cleanup) the head and cylinder look fine. The piston is a Hepolite marked with 7892-1a and 71 H underneath. The top of the piston has 010 in fairly large text, and STD in smaller letters in the same spot as the 010. It seems like if I could find another of these pistons the repair could be fairly cheap with new rings and gaskets. I haven't found any of the 7892-1A pistons, only the 7892-1B. Anyone have a source for these?

We haven't found a definite cause for this yet but think that maybe the timing advanced too much. It was running the original points ignition. My son bought me a Boyer ignition for Christmas that is still in the box.

Any help would be appreciated. Thanks for looking.

Wade


1970 BSA Thunderbolt
1974 Moto Guzzi Eldorado
1977 Yamaha XS650D
1981 Yamaha SR500H
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Re: Source for Hepolite 7892-1A Pistons?
Wade Dunham #795050 01/07/20 2:41 am
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I would be concerned about the debris that fell in the bottom end..

Perhaps replacing both pistons at the same time would be quicker and advantageous


[Linked Image from live.staticflickr.com]
Re: Source for Hepolite 7892-1A Pistons?
C.B.S #795189 01/08/20 7:57 pm
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Thanks for the reply. After much Googling it appears that I don't have a choice. On Friday I'm going to take the cylinders to a local machinist that my son and I have used for several bikes, most recently his 1961 A10, to have him measure the bores. I may be knocking at your door very soon!


1970 BSA Thunderbolt
1974 Moto Guzzi Eldorado
1977 Yamaha XS650D
1981 Yamaha SR500H
Re: Source for Hepolite 7892-1A Pistons?
Wade Dunham #795202 01/08/20 9:20 pm
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Wade: Cleaning out the entire engine would make sense after a melted piston. A set of new pistons of some kind with rings included will make more sense once you see the price a set of rings.

Gordo


The roadside repairs make for the best post ride stories.
Re: Source for Hepolite 7892-1A Pistons?
Wade Dunham #795470 01/12/20 6:55 pm
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It appears that I have more time to figure out what pistons I'm going to use. Also looks like the oil in the tank all drained to the sump and floor...

[Linked Image from live.staticflickr.com]

[Linked Image from live.staticflickr.com]

[Linked Image from live.staticflickr.com]

Wade


1970 BSA Thunderbolt
1974 Moto Guzzi Eldorado
1977 Yamaha XS650D
1981 Yamaha SR500H
Re: Source for Hepolite 7892-1A Pistons?
Wade Dunham #795475 01/12/20 7:42 pm
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Hi Wade
Some years ago I rebuilt a customer T140 that had holed a piston
In the short amount of time it took to get the bike to the roadside the shrapnel from the piston
had travelled all the way through the oil system resulting in the need for a full strip down and thorough cleaning
The roller main bearings produced some beautiful alloy "snowflakes" from the particles of the piston !!!!!!!!
I replaced all the engine bearings as a precaution
John

Re: Source for Hepolite 7892-1A Pistons?
Wade Dunham #795615 01/14/20 8:53 pm
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man you really need to split the cases clean out the sludge tube, and take oil pump apart ck. for damage, pin bushings and big end/brgs.?? you got a mess

Re: Source for Hepolite 7892-1A Pistons?
chaterlea25 #795641 01/15/20 2:09 am
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I wasn't expecting to find that much debris, there are some big chunks of piston in that mess! Looks like I'm going to be doing the same thing you did. I figure the oil tank needs to be cleaned as well, and when it goes back together it will get an external oil filter. On the bright side, at least I'll know it has a clean sludge trap!

Wade

Originally Posted by chaterlea25
Hi Wade
Some years ago I rebuilt a customer T140 that had holed a piston
In the short amount of time it took to get the bike to the roadside the shrapnel from the piston
had travelled all the way through the oil system resulting in the need for a full strip down and thorough cleaning
The roller main bearings produced some beautiful alloy "snowflakes" from the particles of the piston !!!!!!!!
I replaced all the engine bearings as a precaution
John


1970 BSA Thunderbolt
1974 Moto Guzzi Eldorado
1977 Yamaha XS650D
1981 Yamaha SR500H
Re: Source for Hepolite 7892-1A Pistons?
Wade Dunham #795699 01/15/20 6:49 pm
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MAP Cycle St. Pete Fl. has a nice remote oil filter kit uses a Trident cartridge filter. Along with the oil-bag make sure you flush/replace the oil lines. Happy cleaning!!!


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