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Info Aliexpress AHL pistons and shells information, clearances etc - FZR250R 3LN1

Discussion in 'Yamaha 250cc In-Line 4's' started by manynathans, Dec 15, 2020.

  1. manynathans

    manynathans Guest

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    As part of my 3LN engine rebuild I ordered a new set of pistons, main bearings and big end bearings from AHL on aliexpress. The mains and big end bearings were so cheap it was worth buying them to measure them up properly and see how good things were etc.

    This engine has just under 21000 miles (33600km) on it and was stripped due to a sprocket nut failure on the output shaft.

    The original main bearings in my engine looked good and measured within spec for new parts so I was not going to replace them. My engine had 4 black and 2 brown bearings fitted. I test fitted the aliexpress parts and measured the clearances with plastigauge. The bearings all measure consistently for size when compared to each other and the lettering on the back of each shell even matches that of the original yamaha bearings. However all 6 of the AHL bearings measured as 0.07mm clearance. This is higher than the standard value of 0.023 - 0.047mm and getting very close to the wear limit of 0.08mm.

    The big end bearings need replaced on my engine which has 3 blacks and one brown fitted. After lots and lots of searching I was able to track down some genuine yamaha ones to fit, however I had also ordered a set from AHL in case I wasnt able to find enough genuine ones. I will measure these in the next few days and let you know what they are like.

    Last up is the pistons and rings. The bare pistons are somewhere between 3 and 4g lighter than the original yamaha ones, its hard to be exact because there is a fair bit of carbon buildup on the used yamaha ones.

    The gudgeon pins are on average 0.4g lighter than the yamaha ones.

    I measured all the pistons in the bores (which had just been honed due to the engine lying apart for 20 years) and the piston to bore clearance came in between 0.055 and 0.06mm which is the yamaha specified limit.

    The ring sets were mixed and matched to get the most consistent clearances across the cylinders. Worth doing as otherwise I would have ended up with a top ring being a bit on the tight side in cylinder 4 and the same with one of the oil rings. The chart below shows the clearances I was able to come up with after mixing and matching.

    The manual spec is 0.15-0.3mm for the top ring with a 0.8mm wear limit. Without mixing and matching the rings some would have come up as 0.3mm so it is beneficial to take the time to measure every ring in every cylinder and note down the results to come up with the best combination. Oil ring clearance should be between 0.2 and 0.7mm, once again the AHL rings did well in this test with all of them being at the tighter end of this spec.

    The second rings however are a little different and I still havent made a decision on what to do with these. The 2KR manual states 0.15-0.3mm clearance just like the top rings, however the 3LN supplement states 0.3-0.45mm I dont have any 2KR yamaha rings to compare with my 3LN rings to see if there are any obvious differences in their construction. All I can think of is that the 3LN models had a higher redline than the 2KR. So if the china rings were to be treated like 2KR rings then yes they are all within spec, however if they are to be treated like the 3LN rings which is the bike the pistons are for then they are all too tight and will need a little filing to take them back up to a 0.3mm clearance. I will have to give this some thought.

    .................Top Ring Gap / Second Ring Gap / Oil Ring Gaps
    Cylinder 1........0.20.................0.25..............0.25,0.25
    Cylinder 2........0.20.................0.25..............0.25,0.20
    Cylinder 3........0.20.................0.25..............0.20,0.20
    Cylinder 4........0.15.................0.20..............0.25,0.25

    Finally after all the mixing and matching I was able to come up with accurate weights for each piston assembly. Therefore each complete set consisting of piston, rings, gudgeon pin and circlips was weighed as an assembly to find out the final weight difference between them. The results are below. The lightest assembly is used as a reference weight so that the heavier ones can be made to match that. For engine building 0.1g is probably as close of a difference between the heaviest and lightest assembly as anyone would need. No need to pull your hair out trying to get the last hindredth of a gram exactly matched. When I get time I will get the carbon off the original pistons and weigh them as well for comparison.

    ....................Piston Weight Amount Of Weight To Be Removed
    Cylinder 1..........88.64g.............................0.66g
    Cylinder 2..........88.62g.............................0.64g
    Cylinder 3..........87.98g.............................0.00g (lightest piston)
    Cylinder 4..........88.36g.............................0.38g

    I hope this information is of use to people. So in conclusion:
    AHL ring fitment is generally on the lower side of tolerance, best to measure as there is a chance one or two may end up too tight. The rings themselves look of decent quality and feel very similar to the yamaha ones. Not like some of the super brittle china rings I have encountered before.

    The pistons are also respectable and excellent value for money, I am also happy to use these in my engine.

    The main bearings are usable if you are stuck and cant source genuine replacements, however you are essentially replacing one worn bearing with another if you use these due to the very loose clearances and it will always be better to try and find genuine.

    The rod bearings have yet to be compared.
     
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    Last edited: Dec 15, 2020
  2. manynathans

    manynathans Guest

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    Weighed the original yamaha pistons today. There is 0.1g of a weight difference between the heaviest and lightest one.

    Then took some weight off the aliexpress pistons. There is not a lot can be done with the pistons themselves 0.3g was the most I could take off them from the thickest parts around the pin area. Any more and I would have started eating into material that I would not be comfortable removing. So the rest of the weight came from the inside of the gudgeon pins, these are steel so it takes a lot less material removal to get the weight down. Ended up with 0.02g between the heaviest and lightest piston assemblies.
     
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  3. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    This info may help you with bearing clearances for mains and big end

    FWIW, I've seem someone measuring out the mains for a Subaru engine on youtube, first step - inside micrometer or bore gauge for the mains in the casse, then measure the journals themselves, and the final step was the shell thicknesses in the bottom centre with a dial micrometer on a stand

    Hit the calculator for results

    Only really requires one bore gauge, external micrometer to measure the bore gauge and measure the journal and a dial micrometer on a stand for the shells

    https://www.2fiftycc.com/index.php?threads/fzr-250-overbore-300.10253/page-2#post-124506
     
  4. manynathans

    manynathans Guest

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    I fitted the AHL big end bearings today and checked the clearance. I have 3 black and 1 brown bigend bearing in my engine. The AHL bearings gave me around 0.05mm clearance. Just like the main bearings, this would put them out of spec as yamaha state it should be 0.021-0.04 for the big ends with a wear limit of 0.08mm. At least they appear to not be as bad as the AHL main bearings. I took them back out and fitted the genuine yamaha ones which I had managed to source instead which gave me close to the 0.03 clearance that I was after. The AHL parts can be kept aside for if I ever have to rebuild another one of these engines and cant source any more genuine parts. Although it would probably be wiser to buy their undersize mains and get the crank reground to suit. At least that way it would end up with respectable clearances.

    The bottom end is now back together anyway. a.jpg
     
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  5. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    This is a good video on plastigage, shows it's utility AND it's shortcomings

    for the FZR250 with split crankcases, there just isn't the possibility to close and hone main bearing caps as a simple exercise to correct for errors in linearity

     

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