piston problems

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ironbutt
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piston problems

#1

Post by ironbutt »

hey folks,
got a shovel question for you guys, i just got back a bottom end of a 74" shovel ,the shop put it back to stock with all new parts. the guy has been building these motors for 30 + years so i trust him. i bought the motor in peices. i put the pistons on the rods and when it is on the down stroke the piston skirts hit the fly wheels. the pistons are from superior and are standard size for a 74" shovel. the shop did not see anything out of the ordinary with the rods or flywheels when he did it. but he was assuming all the parts were correct. what could be happening ? stroker ? wrong rods ? wrong pistons ? the stroke coming up is fine the piston tops out where it should its just going down that there is a problem. what should i be looking at. i hate to have to split this thing again. any remedies, fixes or ideas would be appreciated.
thanks, dean
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Re: piston problems

#2

Post by biker_k »

In my head, your solution is fairly evident. Pistons that have shorter skirts. No way do you want to have the piston skirts touch the flywheel. You can't buy longer rods if the stroke is correct, so you have to install pistons with shorter skirts but the correct dimension from wrist pin to the top of the piston.
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Re: piston problems

#3

Post by steinauge »

Looks like a set of -74 rods with -55 type pistons.In 1974 HD started using a rod that was .030" shorter CtoC.The new style -73 piston had the wristpin hole lowered .030" to allow for this.If the pistons you are using are the -55 dimension they may well touch the flywheels or the skirts may touch at BDC.If this is the case just trim the piston skirts to allow .030-.060" skirt to flywheel clearance and dont sweat it.If you are thinking about the CR change it amounts to less than 1\2 point.
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Re: piston problems

#4

Post by jjohnson1177 »

You could also have 80" flywheels with 74" pistons. The longer stroke on the 80" wheels would jam it up in a hurry. It's going to be to much to grind on the piston skirts to get it to clear plus it will mess up the balance.
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Re: piston problems

#5

Post by steinauge »

If the engine was 41\4" stroke the piston would come out further on the top too.I believe the statement was that the problem was on the BDC side not TDC.
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Re: piston problems

#6

Post by ironbutt »

okay guys,
got my head cleared and here is what i have. i rechecked the stroke and it is 4 1/4, so i am thinking the wheels are from an 80 " shovel. i rechecked the TDC and it is coming up farther than it should also ,the top lip of the piston is flush with the top of the cylinder, should be below. what i have is a 1973 74 " shovel with 80" wheels and brand new 74" cylinders and pistons. now these pistons will not work i would have to mill to much off of them, so any ideas on pistons that would work that are for a 3 7/16 bore 74" motor that has a 4 1/4" 80" set of flywheels in it. i would like to be able to use my 3 7/16 jugs as they are brand new and just buy new pistons.
hope i made this a little clearer.
thanks
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Re: piston problems

#7

Post by RUBONE »

Ironbutt,
I answered this once for you but here goes again;
That combination has existed since the first guy put UL flywheels in his Knuck. Pistons have been available almost as long. Truett and Osborne in Wichita,KS, S&S in Viola, WI, Dixie Distributing, I suspect V-Twin, J&P, etc, all would have pistons for your application. No big deal. Some of them will have options on compression ratios. That is your choice. A standard 80 stroker piston is for a 3 7/16 bore. 80 inch Harley motors are 3 1/2 or the same as .060 over. H-d did that to get a true 80" motor. Harley ULH 80" motors are about 1275cc.
Robbie
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Re: piston problems

#8

Post by ironbutt »

wow, settle down there cowboy. didn't mean to make you repeat yourself. if i understoud what was going on here i would'nt have come to you guys for some answers
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