Piston Too Tight In Bore

massacre

massacre

#1
#1
Hello,

I have a bigger bore HS50 where the bore was worn so I bought a .010 over thick ring piston and rings for it. Brought block to machine shop with piston and rings and they bored it out.
After installing the piston and putting everything together, the piston is too tight in the bore. Or should I say, the rings are too tight, the piston with no rings seems fine.
I’m not sure if the machine shop honed the block afterwards, but I didn’t before assembly.

When I put the top ring in the bore, there is like an 1/8” ring gap which to me seems wrong. Seems like a huge gap would be looser, not tighter. So I am guessing that I have the wrong rings.
I have a smaller bore HS50 piston here with thick rings, when I put one of those rings in the bore the gap is much better.
If I use the rings from the smaller piston and everything seems good, is that ok to run the motor with?
Thanks
 
delray

delray

Well-Known Member
#2
#2
your sounding little confusing. every big hs50 engine I have seen comes with the thick rings and ever small bore hs-50 2.795 bore comes with thinner rings. you need to double check your part numbers you got for those rings and you need to measure piston and the bore to see if that stuff is correct. best thing to do if you can't figure things out. take it all back to the machine shop and just ask them to look at it real quick. they will figure it out for you about 5 mins or less.
everybody here can give there opinions,but we can't see it in front of us or measure anything to really give you the right answer. back to the machine shop and it will be all good.
when you installed the other rings the gap was better? what was the gap?
 
massacre

massacre

#3
#3
Ok thanks Del Ray

I’ll remeasure everything again just to make sure.
The rings came in box with the piston but I bought it off eBay so who knows if they are the correct rings or not

I think I may have got my parts confused, there’s stuff everywhere lol

My question is I asked them to bore it but not sure if they honed it.
Should I try to hone it? I have a decent drill press here
 
delray

delray

Well-Known Member
#4
#4
NO...……….DON'T HONE IT YET. they should of done it already. need to measure things before doing anything else or you could have even bigger problems. engine is not worth rushing over. get it measure first then you will know where to go from there. you should be able to see a good cross hatch from the machine shop. it needs to be a 400 grit finish for Tecumseh rings. Tecumseh books actually say 390 grit, no one has that grit? and 400 will work just fine. engine won't know the difference.
 
C

cfh

Well-Known Member
#5
#5
You can’t trust part numbers on eBay either. I’ve bought tons of pistons and rings where they claim, for example, it’s a part number for thin rings but clearly in the picture it’s thick rings. They often get these things wrong.

And on HS 50 motors you can’t confuse these things. Thin rings and thick rings/pistons are not interchangeable

Now on HS 40 motors you can interchange thin and thick ring pistons. But you can’t do it on HS 50 motors

My current approach to rings is to always buy either .010 or .020 over rings. then trim the ends to get the exact ring gap I’m looking for. I shoot for .009 ring gap, but find you can go up to .019 Without any issues. ( because sometimes I over grind. Yeah I know but it happens)

Basically nearly all HS 50 motors are going to be thick ring. HSK50 motors will be thin ring.
 
delray

delray

Well-Known Member
#6
#6
cfh said:
You can’t trust part numbers on eBay either. I’ve bought tons of pistons and rings where they claim, for example, it’s a part number for thin rings but clearly in the picture it’s thick rings. They often get these things wrong.

And on HS 50 motors you can’t confuse these things. Thin rings and thick rings/pistons are not interchangeable

Now on HS 40 motors you can interchange thin and thick ring pistons. But you can’t do it on HS 50 motors

My current approach to rings is to always buy either .010 or .020 over rings. then trim the ends to get the exact ring gap I’m looking for. I shoot for .009 ring gap, but find you can go up to .019 Without any issues. ( because sometimes I over grind. Yeah I know but it happens)

Basically nearly all HS 50 motors are going to be thick ring. HSK50 motors will be thin ring.
Click to expand...
HSK50 is the small bore block correct. (2.795) that would make sense why the rings are thin.
 
massacre

massacre

#8
#8
I think the “.010 over” rings might not be the right ones.
They are the ones with the huge ring gap.
I def will not hone it.
Thanks everyone for the helpful replies, i’ll Get everything measured and figure this out.
 
massacre

massacre

#10
#10
So I think I know what happened, I think I will have to go back to the machine shop.

Stock tec spec BB piston is 2.812”
Stock tec spec SB piston is 2.795”

My non oversized piston is measuring 2.7920”
Oversized is 2.7955”

2.7920” + .010” = 2.802”, not 2.7955” which leads me to believe I had the wrong piston this whole time.
even stock 2.795” piston + .010” = 2.805”
My bore is now at 2.819”

Now my question is what is the clearance between the piston size and the bore size?
I don’t think 2.819” is big enough for a 2.812” piston.
But maybe it is?

If I know what the clearance should be, I can know whether I need to either get a different piston or bore this block out a little more.

Any info would be great thanks
 
massacre

massacre

#11
#11
If stock BB bore is 2.8125”

2.8125 + .010” = 2.8225

My bore is 2.819” which says to me that the bore should be bigger than it is now

I think now maybe the bore is too big for the piston(s) I now have, which might be rocking the piston in the bore, making it feel “tight” but is actually loose and rocking side to side.
 
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massacre

massacre

#12
#12
I’ll get the correct piston and report back. I have a ton of HS50 stuff here and I may have got them confused lol
Good thing I am not saving lives over here

It’s my own fault for buying stuff from eBay and not checking it first.
 
massacre

massacre

#14
#14
Looking for specs for piston to wall clearance on HS50
Trying to find this info online but coming up with nothing specific.

If I can find out the specs then I can figure out my next move
Thanks!
 
C

cfh

Well-Known Member
#15
#15
It's really easy to tell the thin versus thick ring pistons apart. In the case of HS50, the thick ring will be the larger (old style) size. The thin ring will be the smaller (newer style) size. The cut out around the wrist pin is the key. If you see the cut out, it's the newer style smaller HS50 bore. Here's a picture to help...

 
massacre

massacre

#16
#16
I have 3 pistons here, none of them have the cutout.
They all measure out to be about 2.795 ish.
I guess I am confused since I thought the big bore piston is supposed to be 2.812

Am I wrong on this?
 
massacre

massacre

#18
#18
So I am starting to feel dumb.

Is the bore 2.812 and the piston 2.795 for a piston to wall clearance of .017?

If so I aplogize for wasting everyone’s time.

If that is the case, my bore is 2.819 which means the piston to wall clearance is .024
That seems like it is too much
 
Last edited:
massacre

massacre

#19
#19
Than means 2.795 + .010 = 2.805

2.819 - 2.805 = wall clearance of .014 which sound much better.
Now I need a 2.805 piston which I thought a .010” over Hs50 piston was

Am I even close on any of this?
 
delray

delray

Well-Known Member
#20
#20
ok lets get something straight first. the 2.795 is for the newer hs-50 and ohh bores. the 2.812 is for the old style hs-50 points and just couple years into the electronic ignition when it started.
and when it comes to piston way they are machine/made. they can come in all different designs for the 2.795. I have seen at least 3 different designs. but for the old style big bore? only one for me so far. I give it a nick name....lol slug piston out of a hit and miss engine...lol. big and ugly with thick rings...lol
 
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