Sprite Race Motor Rebuild

Discussion in 'Classic Mini' started by Minidave, Mar 1, 2021.

  1. Minidave

    Minidave Well-Known Member
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    #21 Minidave, Mar 12, 2021
    Last edited: Mar 12, 2021
    They really are.....

    Kent Prather is a real guru on these little Sprite motors, his latest version is making 165 hp at the flywheel on the dyno......

    Of course, that's a $15K motor too.....or more!
     
  2. MCS02

    MCS02 Moderator
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    It’s looking really good so far. You said your boring it 60 over. How big can you go? Also can you sleeve these engines if you want to go back to stock size pistons?
     
  3. Minidave

    Minidave Well-Known Member
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    Actually, these custom race pistons are 70 thou over - you can go bigger, but then you have to offset bore. You can sleeve them back to the stock bore, but you can't really overbore the sleeves once they're in place - you're stuck with stock bore - which is fine of course. 120 thou over (offset) give a 1380cc engine. I'll post up some pics of these pistons with their Teflon coated skirts....
     
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  4. MCS02

    MCS02 Moderator
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  5. Minidave

    Minidave Well-Known Member
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    #25 Minidave, Mar 14, 2021
    Last edited: Mar 15, 2021
    The owner knew that his engine was running pig rich, but when he tried to adjust the mixture (done by raising or lowering the jet on a SU carb) the jet wouldn't stay up. On looking at it I could see that the choke linkage was holding the jet down and that it was bent in all sorts of weird ways - that's how the choke works - it drops the jet down to richen the mixture and the return spring in the linkage draws the jet back up and holds it against the adjuster. So I took the link off and reconfigured it and viola! now it works!

    I had to straighten the bend on the end where it attaches to the jet, square the first bend coming off the linkage as it was only about 60*, then re-bend the jet end in a different place to shorten it. It was actually a little tough to do as this metal is pretty stiff, I couldn't bend it with pliers, I had to use a hammer and the vise. Which tells me someone bent it that way to begin with, and/or these carbs have been messed with like everything else on this engine so far.

    IMG_20210314_164120R.jpg

    IMG_20210314_164127R.jpg

    One more thing off the list.

    Still waiting for main bearings and rings so I can build the block. The head is done, I rebuilt the rocker arm assembly with a new shaft and refaced the surface of the rocker that hits the valve, so the head is ready to go on when the block is done. This rocker assy is a short term solution, when we re-do with the head later in the year or post season with new hardened seats and new valves we'll put some new roller tip rockers on it.
     
  6. Minidave

    Minidave Well-Known Member
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    Well, as often happens when you want to get things done quickly....parts orders get screwed up and sent to the wrong address and so on ....bottom line rings and bearing won't be here till Thursday at the soonest, so there's no way we can make the race at Hallett this weekend. Too bad, since it's close enough I might have been able to go....still might but it's unlikely at this point.
    The next race is at Gingerman, in Michigan, towards the end of April, so that gives us more time for break in and tuning. I won't be going to that one, it's more like a 10-12 hour drive and I doubt I'm up for that - but you never know.
     
  7. 00Mini

    00Mini Well-Known Member

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    Bummer that you had a parts snafu so very close to the end but as you say you’ll have more time for break in and tuning.
     
  8. Minidave

    Minidave Well-Known Member
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    Forgot to post a pic of these custom made 70 thou over forged race pistons....the black on the skirt
    is a specialized teflon coating
    IMG_20210316_160444.jpg
     
  9. 00Mini

    00Mini Well-Known Member

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    Impressive and no doubt pricey.
     
  10. Minidave

    Minidave Well-Known Member
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    Well.....another delay on this build, they sent the wrong rings. So, another call and another set on the way, we'll be lucky if we get them by next weekend!
     
  11. MCS02

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    I hate that kind of stuff. Little mistakes that cost you time.
     
  12. Minidave

    Minidave Well-Known Member
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    Just got a call, turns out the rings are correct after all, the package was labelled wrong, so it's on again. I'll get them tomorrow....
     
  13. 00Mini

    00Mini Well-Known Member

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    Anyone who does a lot of work on things has experienced the mislabeled package correct part or wrong label package correct part and fully understands the frustration it can cause.
     
  14. Minidave

    Minidave Well-Known Member
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    #34 Minidave, Mar 20, 2021
    Last edited: Mar 27, 2021
    OK, so this is a new one on me - probably because when I was building Sprite racing engines (Team name - Impecunious Racing) in the late 60's they had no such things!

    Anyway, the top ring is a standard looking ring, but the second one.....there is a fairly normal looking ring, but on closer inspection the is a dado in the inner part of it, and in the package - a second ring that goes inside the first, and fits in the dado! It should be interesting trying to fit them as the gaps are supposed to go opposite each other. So I guess fit the inner ring on the piston, then the outer - rotate them and hope that when I use the ring compressor they stay together the way they're supposed to!

    I've looked them over a bit, and I don't really see how they go together.....but I'll study them some more.....

    IMG_20210320_183239R.jpg
     
  15. Minidave

    Minidave Well-Known Member
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    After I cleared out a few other projects and did a bit of cleanup and organizing, today I pulled the Mini engine out of the test stand and installed the brackets for a Sprite motor.

    IMG_20210326_123107R.jpg

    Next up I laid out all the rings so I could custom cut the ring gaps - since this is a race motor Kent (who supplied pistons and rings) said he wanted 15thou endgap rather than the usual 8-12, so each of the top and second rings had to be filed slightly and re-measured several times till just right, and any burrs carefully cleaned off of the edges.

    IMG_20210326_151509R.jpg

    With that done, next up I cleaned up the crank journals and put in the new main bearings, then laid the crankshaft in. Next I installed the thrust washers, then the studs for the mains, then the center main cap got torqued down. The crank turned smoothly so I did the same with the other two mains one at a time, to make sure if there was any binding I knew where it was. The crank rolls smoothly, so all good there.

    IMG_20210326_162445R.jpg

    The next job was to mount up my dial indicator and check the end play - it should be around 3-5 thou and it was spot on at 3 thou.

    The next job will be to install the pistons without the rings and then check all measurements, plastigauge the clearances and check the piston to deck height. When I tore it down 3 of the 4 pistons were proud of the deck by a few thousandths, given how the rods can stretch under high RPM use, you don't want them hitting the head or creating any more "crop circles". If we have that situation I may need to send the pistons off to machine shop to have a little taken off the tops. To that end, I have the old head gasket and I will put some clay on top of the pistons, install and torque the head down and roll it over a few times, then remove the head and see how much room there is - the owner found a deal on some 1.5 roller rockers that he wants to use so I need to make sure the valves and pistons won't meet up!

    I also have to CC the heads and then try and compute the compression ratio, he thinks it's about 13:1, and we sure don't want it any higher!

    So, still lots to do before it will make noise....his next run is the weekend after our Texas trip so I still have a lot to do.
     
  16. Minidave

    Minidave Well-Known Member
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    #36 Minidave, Mar 27, 2021
    Last edited: Mar 27, 2021
    Today I did a "dry build", where you put the pistons in without rings so you can measure a bunch of things like rod bearing clearance and how far the pistons are above or below the deck surface. When I tore this engine down one piston was 3 thou below and the others were about 5 thou over the top of the deck.

    IMG_20210327_114906R.jpg

    Today they are all uniformly 25 thou below the deck, which will make all my other calculations much easier - I don't have to worry about the pistons hitting the head or the valves hitting the pistons.

    IMG_20210327_145738R.jpg

    With that done I installed the rings onto the pistons and put the pistons into the block, then bolted up the caps and torqued them down. Everything still turns nice and smoothly so I put the oil pump pickup tube in and installed the oil pan. Next I flipped the motor over so I could set up the dial indicator and set TDC, then set up for checking the cam timing.

    IMG_20210327_152629R.jpg

    IMG_20210327_153807R.jpg

    I really need to cc this head, as the calculations I'm coming up with indicate a pretty high compression ratio.....
     
  17. Minidave

    Minidave Well-Known Member
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    BTW, that's a vernier adjustable timing gear.....
     
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  18. Minidave

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    Today I had PT and as usual I was pretty whipped when I got home, but I did get a little work done on the engine. Last night I mocked up a few things to make sure they would work in the engine stand. For example since the oil cooler and assorted hoses stayed with the car I had to rig up a standard spin on oil filter setup so I can run it in the test stand. I wasn't sure the oil filter was going to clear the extra oil pan square.....which it does.

    IMG_20210329_091448R.jpg
    IMG_20210329_091438R.jpg

    I also put the the head on, and found that the extra two studs must have been drilled by hand, they are a long way from square to the block surface! They will work, but I have to put the head on, then insert the studs.....

    Then I moved around the the clutch end and found the pilot bearing was worn badly. The trick to getting one of these out is to fill it with grease, then inset the input shaft just started into the bushing and give it a good smack with a dead blow hammer - the hydraulic effect will force the bushing out of the hole. However, this one was so worn out that the grease would have just squirted out around the shaft sooo......I found a piece of allthread that was a little bigger, and carefully ground it down till it just fit the bushing. Then I did the grease trick and it came out - in three pieces. i think the hydraulic force broke it up.

    IMG_20210329_160118R.jpg

    Anyway, the new one went in without any drama, so I installed and torqued up the flywheel, then installed the clutch assembly, using an old input shaft as an alignment too.

    IMG_20210329_160811R.jpg

    IMG_20210329_164218R.jpg

    I also called the company in California that supplied this cam and got the scoop on where to set the cam timing - where I might usually set one up at 106* ATDC, they recommended 103 for this scatter cam - glad I called.

    More as it happens.....
     
  19. MCS02

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    Nice work Dave!
     
  20. 00Mini

    00Mini Well-Known Member

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    Did you cc the head ??
     

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