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Briggs Engine Rebuild Help


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#1 petrj6 OFFLINE  

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Posted May 04, 2014 - 06:03 PM

    I need a little help here, I am rebuilding a brigs 243400 series engine.  I had to go .020 over with the piston and now am having trouble getting it back together, A very good friend of mine bored the engine for me, he said I wanted about .002 clearance for the piston in the bore, so that is what I did, I honed till it was correct, the piston went down the bore fine with just a little effort, when I went to put the engine back together again the rings were way to tight, I honed a little bit more on the bore but the rings are still to tight.

  I looked up the ring cap tolerances and they should be around .012 for that size bore but I wanted to be sure about the bore size, I am not finding much on the net about it for air cooled stuff, the piston is 3.825 and the bore around 3.855. 

   Any one have any tips or knowledge of this?



#2 New.Canadian.DB.Owner OFFLINE  

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Posted May 04, 2014 - 06:38 PM

Maybe my math sucks (again), but 3.855 - 3.825 = .030.  That is way more than .002.  Any chance you missed a zero, as in 3.0855?

 

briggs cylinder sizes.jpg

 

Here is what the manual says on piston re-sizing:

 

 

RESIZING CYLINDER BORE
ALWAYS RESIZE TO EXACTLY .010” (.25 mm), .020” (.51 mm), OR .030” (.76 mm) OVER STANDARD BORE SIZE AS SHOWN IN TABLE NO. 1, SPECIFICATIONS. If this is done accurately, the stock oversize piston and rings will fit correctly and proper clearances will be maintained. Cylinders, either cast iron or aluminum, can be accurately resized with hone sets listed in TABLE NO. 8, Specifications.


Edited by New.Canadian.DB.Owner, May 04, 2014 - 06:43 PM.


#3 petrj6 OFFLINE  

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Posted May 04, 2014 - 06:50 PM

    Rite, my mistake. size would be 3.0825 :thumbs: :wallbanging:  It has been a long day,  Thanks for the input.  I have read that about going exactly .010 over and the clearances should be in the piston, the trouble I am having is if I went .010 over that would put the bore at 3.0825 and the .020 over piston came in exactly at 3.0825.  They cannot run size to size can they?  the piston would not even go down the bore that way, I had to give it .003 clearance just to get the piston down the bore.


Edited by petrj6, May 04, 2014 - 06:57 PM.


#4 New.Canadian.DB.Owner OFFLINE  

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Posted May 04, 2014 - 07:06 PM

According to the manual, .003" bore oversize is pretty much max.  What is your ring gap?

 

briggs piston inspection.jpg briggs piston inspection 2.jpg

 


Edited by New.Canadian.DB.Owner, May 04, 2014 - 07:08 PM.


#5 WNYTractorTinkerer OFFLINE  

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Posted May 04, 2014 - 07:08 PM

Just to clarify what I think you're doing..  You used a hone to bore out your cylinder?  OR did you use a boring tool?  

 

.002 clearance from the piston to thebore is way tight unless it's a water cooled model..  



#6 New.Canadian.DB.Owner OFFLINE  

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Posted May 04, 2014 - 07:23 PM

    Rite, my mistake. size would be 3.0825 :thumbs: :wallbanging:  It has been a long day,  Thanks for the input.  I have read that about going exactly .010 over and the clearances should be in the piston, the trouble I am having is if I went .010 over that would put the bore at 3.0825 and the .020 over piston came in exactly at 3.0825.  They cannot run size to size can they?  the piston would not even go down the bore that way, I had to give it .003 clearance just to get the piston down the bore.

Okay, assuming you are playing with a cast iron block and the .010" piston/bore clearance, the standard size bore is 3.0615" to 3.0625", making your standard size piston 3.0515" to 3.0525".  At .020" over, the bore should be 3.082 and piston should be 3.072".  A .030" over piston would be 3.082".  Does your piston have a stamp saying +0.020" or something similar?



#7 petrj6 OFFLINE  

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Posted May 04, 2014 - 07:29 PM

   Ok, I had the engine bored out on a machine where I work, I got a new piston .020 oversize it was all I could get for an oversize piston.  there was a ridge in the bottom of the bore that I had to get rid of.  we measured the new piston and bored the engine to that size 3.085, then I honed out .002 for a clearance making the fineshed bore 3.088, now the rings are to tight and I need to cut them down but before I do that I wanted to be sure I had enough clearances for the piston.

   I was talking to another friend today who builds race engines and he said I needed more clearances for air cooled engines, so before I cut the rings I wanted to be sure about the bore and clearances



#8 petrj6 OFFLINE  

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Posted May 04, 2014 - 07:45 PM

   I am sorry guys if I get the numbers a little screwed up, I am not a machinist but I am trying to learn as I go.  What I really need to know is what would be good to have for clearances on the cylinder, it is an aluminum piston in a cast iron block,  I had to go .020 over due to a lack of parts, .020 over was all I could get.  And the .020 over piston came in rite to size 3.0825 and yes it is stamped .020 over on the piston.

   This has been a huge headache all the way thru, when I first started I got a .010 over new old stock piston and when it came in it was rite on size for stock bore, I had to buy a .020 over piston just to get one that was .010 over the stock size.  I know it sounds strange but this motor has been a huge hassle to get done, I thought I would have it running today but I guess not.






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