Author Topic: Stationary engine rebore  (Read 699 times)

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Offline TonyC

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Stationary engine rebore
« on: 08 Nov 2023 at 20:50 »
Hi all,
Can anyone give me some advice on doing a re bore on a single cylinder SV54. My engine is very worn so I bought a new piston which is 20 thou up on size, but although being advertised for a SV54 is 3 1/2” diameter whereas mine is 3 1/4” diameter. I have a spare engine to use as spares and taking this one apart found the bore is 3 5/16” but the rings are seized on the piston. So my plan is to bore this cylinder out to suit the new piston as I won’t need to take so much out and I still have the original if there is a problem. Checking the wall section, once machined I would have a wall section of 4 mm. So is this sufficient? and then in my ignorance of never doing a rebore can I assume that the bore is machined out to the exact size of 3 1/2 plus the 20 thou.
Hope someone can help before I start machining!

Tony

Offline EW-Ron

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Re: Stationary engine rebore
« Reply #1 on: 08 Nov 2023 at 23:11 »
Whoa   !!

Where/how did this +0.020" come into it.
Thats the size up (oversize) from the previous piston.

The running clearance of the piston in the bore is likely to be less, much less.
0.005" or maybe 0.006" is likely to be typical.
It should be quoted somewhere - instruction sheet with the new piston ?

So to be clear, the bore would be your 3.5 inches plus an extra 5 thou for clearance.
IF your new piston is exactly 3.500"  at the lower skirt = 90 degrees to the gudgeon pin.
Hopethishelps andisclear.

P.S. NEVER bore a cylinder unless you have the piston in hand and are crystal clear on the required clearance.
You would expect to hone out the last few thou - to give the required surface finish.
As Confucious would say ....

Offline TonyC

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Re: Stationary engine rebore
« Reply #2 on: 10 Nov 2023 at 13:18 »
 Thanks for the reply,

The 0.020+ piston was only one I could get.
Measuring the skirt it is 3.512" which gives 0.008 thou clearance on the + 20 thou bore.
Is this too much?

Regards Tony

Offline EW-Ron

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Re: Stationary engine rebore
« Reply #3 on: 10 Nov 2023 at 21:06 »
8 thou does sound like a lot.
Piston slap city ...
Did any instructions come with the new piston. ?

The bore should ALWAYS be done to suit the piston in hand.
Regardless of what nominal size is written anywhere.

If you could end up with ~5 thou clearance after boring and honing,
you couldn't go far wrong ...

Offline TonyC

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Re: Stationary engine rebore
« Reply #4 on: 10 Nov 2023 at 21:21 »
Will take your advice and work to 0.005 thou.

There were no instructions in the box apart from an inspection label from July 1951!

Thank you for your help

Offline EW-Ron

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Re: Stationary engine rebore
« Reply #5 on: 10 Nov 2023 at 23:20 »
Make sure the 5 thou is based on the biggest dimension you can find on the piston.

Usually on the lower skirt, at 90 degrees to the gudgeon pin (ie on the thrust face)

Offline EW-Ron

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Re: Stationary engine rebore
« Reply #6 on: 12 Nov 2023 at 20:16 »
Could be worth tracking down an original handbook for these,
and verifying what the factory recommendation for clearance was. ?