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For the preservation and enjoyment of 1928 to 1931 Indian Scout Motocycles
"You can't wear out an Indian Scout"

37cu in piston

  • 03 Jan 2021 9:34 AM
    Message # 9624408

    I'm looking for a standard or +10thou O/S 37 cu in Scout piston. I need to check the deck heights and haven't a piston. My engine is a collection of parts sourced from the USA. The crankcase and barrels are 1924. I live in the UK and there's absolutely nothing over here. I am more than happy to pay shipping etc. 



  • 04 Jan 2021 7:28 AM
    Reply # 9654957 on 9624408
    Tim Raindle (Administrator)

    Hi Paul, are you looking for a useable piston, or a used one for reference ?


  • 05 Jan 2021 12:41 AM
    Reply # 9679841 on 9624408
    Paul Darby wrote:

    I'm looking for a standard or +10thou O/S 37 cu in Scout piston. I need to check the deck heights and haven't a piston. My engine is a collection of parts sourced from the USA. The crankcase and barrels are 1924. I live in the UK and there's absolutely nothing over here. I am more than happy to pay shipping etc. 



    JP Pistons In Adelaide South Australia   list  Pistons for 37"  Scouts
  • 05 Jan 2021 2:02 AM
    Reply # 9681645 on 9624408

    I'm looking for a used piston. My barrels are just over 10thou oversize at the moment. They will need to be re-bored, so a std or 10thou would fit in.

  • 09 Jan 2021 2:10 AM
    Reply # 9769380 on 9624408

    I guess you are repairing an old clunker and just want it to run. New piston rings is a minimal criteria, but it can be really difficult to find fitting piston rings to an old piston. With new piston rings, it is recommendable to hone the cylinders. Do acquire the piston or pistons first in order to measure and decide piston to bore play.

    It is helpful if you state what piston pin diameter and compression height your other piston has, in order to get roughly the same compression. There was a number of brands of pistons that was used during the years so they can differ a bit between, as can piston pin diameters.

    Especially on an old clunker in for a repair, to have exact same comp. height is not really critical on these low comp. engines. Same with cylinder diameter, 1-2mm does not make a big difference, it'll chug on and run fine anyway but you won't like pair up pistons with too large height difference. A tiny bit of unequal weight is the same as unequal cyl. diam, not a big deal on these engines. 

    To minimise vibration with different brand pistons you can equalise the weights on them by turning the skirt a bit shorter or perhaps minor! adjustment of the comp. height. Avoid machine the inside of the skirt, that can compromise the strength and induce cracks.

    If you are doing new everything up to blueprint specifications, bearings, cams, valves and all that equalised, balanced, tight, spick and span, then the 37ci are wonderful engines, nearly bulletproof and you can beguile them to rev until they are out of breath. Over and over...

    Last modified: 09 Jan 2021 2:19 AM | Carl-Erik Renquist
  • 10 Jan 2021 8:08 AM
    Reply # 9817062 on 9624408
    Tim Raindle (Administrator)

    Carl-Erik, I think the issue is that Paul doesn't have a piston at all, and is looking for a used one to determine the correct dimensions, in order to either have some made or hunt down the correct items, and then he wants to cut it in half and use it as a rod centering tool. 

  • 10 Jan 2021 12:18 PM
    Reply # 9824156 on 9624408


           the project originally came with empty crankcases. I've been able to source 741 flywheels and 37 cu in Scout rods. The cylinders are 1922 (blind) and will need re-boring. I'm ready to assemble the flywheel with all new parts obtained from Walkers. I don't have any pistons at all. I just want to assemble the flywheel with a 37 cu in Scout piston and check the deck height. A standard or +10thou oversized piston will fit into the barrel. I believe the wristpin diameter is .7500"



  • 10 Jan 2021 9:17 PM
    Reply # 9839592 on 9624408

    Aha! but the deck height is easily checked if you assemble the cylinder with a half of the crankcase, then take a strip of paper put it all the way in the cylinder, hold it firmly in place and make a press inprint of the edge of the main bearing bore.

    Then it is just a matter of add the distance to the center of the main bearing bore. Now you know close enough the total height from the crankshaft center to the cylinder head. Subtract half of the stroke and rod length (center to center) from the total height, and what's remaining is the distance from the piston pin center to the cylinder head..

    1mm is the closest the piston should ever get to the top surface, that can be fine tuned with the thickness of the base gasket, but don't be surprised if you find that original pistons turn several millimeters under.

    When I have a chance to tune that distance, I like to have the pistons turn fairly close, say max 1,5mm as with too big distance the combustion tend to be cold and uncomplete on top of the pistons and that induce a large carbon deposit buildup! A coke buildup that eventually makes the pistons hit the cylinder head...and when that happens...

    It happened to me. Suddenly bangbangbangbang, of course it was in the middle of a long tunnel all hell broke out! Sounded like it all came apart and an expensive bottom end restauration was due imminently! I sent for a trailer and took the wreck home...Imagine my relief when I removed the heads and saw it was nothing else wrong than thick ugly layers of oil and soot on the pistons and cylinder heads...

    If I had known better I just had to do as the instruction book says...regularly remove the heads and scrape off carbon deposits. I could have done that on the side of the road in a half an hour and been back in the wind much happier and a fair bit less poor..

    I have to add, that was an old clunker with more issues than one...engines with fresh plugs, round bores and healthy piston rings runs a lot cleaner but regular maintenance does not hurt. 

    In the picture is me having piston ring issues in another engine...I caught it before bangbang happened again, learned at least something from earlier experience... 

    1 file
    Last modified: 10 Jan 2021 10:10 PM | Carl-Erik Renquist
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