So, I have newly re-bored/hohned barrels and a set of +020 pistons. Only problem is that the machinist decide to give me 010 slop! The bores are actually +025. I don't really want to take them out to +040 as this really is a waste of material - anyone have any ideas. Is it possible to "ease" out the skirts 005 without fracturing them or can I get +025 piston anywhere?
Can I get +025 Pistons for an 850?
- Thread starter Nigeldtr
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Most machine shops can Knurl the pistons to make them fit tighter. We used to do it to small block Chevy pistons for drag Racing,
+.030's are pretty easy to get.
Honing out another .005 is almost reasonable. I recall wanting to hone out .010 before and the forum talked me out of it. My .020's were just fine and I had no need to go over at that and this time.
But, .005 maybe OK if a machine does it.
DO NOT, I repeat, DO NOT do it or have it done without the pistons and rings in hand. Bring the bore to the piston size, not the other way around. Keep any variables at bay.
Honing out another .005 is almost reasonable. I recall wanting to hone out .010 before and the forum talked me out of it. My .020's were just fine and I had no need to go over at that and this time.
But, .005 maybe OK if a machine does it.
DO NOT, I repeat, DO NOT do it or have it done without the pistons and rings in hand. Bring the bore to the piston size, not the other way around. Keep any variables at bay.
Thanks for all the replies, I had also thought why not another 005 and then +030 but I could not find +030 pistons for the 850. I'll have another look..
Sadly there are some things I can't do in the home shop and I now hate giving my parts over to someone else. After 40 years there is so much that has to be put right because some lying clueless jerk(s) decided to have a go - sorry another rant. You would not believe the shit I have uncovered.
Sadly there are some things I can't do in the home shop and I now hate giving my parts over to someone else. After 40 years there is so much that has to be put right because some lying clueless jerk(s) decided to have a go - sorry another rant. You would not believe the shit I have uncovered.
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This is how Ms Peel Combat got away essentially Scott Free d/t a bit of set up rust removal honingl left pistons over .006+" loose in .040" bores. Higher end automotive hop rod shops have SUNN Knurling machines - setting amount of knurls proudness by air pressure, Peel's 30-35 PSI. The tips of and all of some knurls will rub off in a few 1000 miles but I did not detect any slap and sure as Hell made more power than I could believe or desire to use much of the time, yet giving her maxed out Hell like the bitch was made for. Can't really be that fond of your craft if you spank em like ya stole em to get some payback.
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Nigeldtr said:
When you say 0.010 slop, is that in addition to required piston-to-cylinder clearance? This clearance figure will depend on what kind of piston you have as different pistons have different compositions and expand differently. If I am remembering correctly, I think a 0.004 clearance was recommended with the Italian GPM pistons I used in my Dominator 88 (500cc).
Well I must confess that I like the idea of Knurling and for 005 surely not a bad idea at all! I would still need to get some bigger rings (+040) and file to fit - in the end it looks like I will have to go out to +040 
Sorry for the confusion, when I say 010 slop, I mean there is 005 too much (allowing for 4 or 5 thou running clearance).
Thanks.
Sorry for the confusion, when I say 010 slop, I mean there is 005 too much (allowing for 4 or 5 thou running clearance).
Thanks.
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When you take the rings that came for your 20 over pistons and put them into the cylinders. Is the ring gap still in spec?
I think that if they were in spec, that I would use them over a ring made for a larger bore that will not be round'
If the ring gap is in spec, I would get the pistons knurled and try it out. It's only a Norton!
I think that if they were in spec, that I would use them over a ring made for a larger bore that will not be round'
If the ring gap is in spec, I would get the pistons knurled and try it out. It's only a Norton!
batrider
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Last time I looked into this it was difficult finding anyplace that still does knurling. It is a lost art that is maybe better off left that way.
The high spots from the knurling (which is what reduces the clearance) will wear quickly and you will be back to square one.
The high spots from the knurling (which is what reduces the clearance) will wear quickly and you will be back to square one.
concours
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No, they'll be a country mile too wide... Whereas circumference equals D x Pi, being .005" big on the bore will give almost .016" extra end gap BEYOND the desired gap.Bruce MacGregor said:
Whether a Norton or a Lawn Boy, shit should be right.
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Throw your piston in the hole . Throw a feeler in at ring lands , CAREFULLY . WHAT thickness is it. PLEASE .
Ditto a Comp Ring at top of bore , a half inch down , squared by piston . Measure End Gap . NOW WE KNOW .
If you tell us . :wink:
Ditto a Comp Ring at top of bore , a half inch down , squared by piston . Measure End Gap . NOW WE KNOW .
If you tell us . :wink:
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Don't sweat the ring gap with standard 20 over set, its long accepted racer practice to get next size up and file gap to spec. We had a deep pow pow last year on this very subject, again.
Knurling is considered a stop gap measure so you're juggle of cost vs endurance vs 040" new bore and piston set. I know of a shop in Arakansas but not hard to knurl. Farm tractors are famous for getting by on knurled pistons so might check tractor shops or local hot rod clubs.
Knurling is considered a stop gap measure so you're juggle of cost vs endurance vs 040" new bore and piston set. I know of a shop in Arakansas but not hard to knurl. Farm tractors are famous for getting by on knurled pistons so might check tractor shops or local hot rod clubs.
Forget most of the dribble, if you insist on using 0.025'' bores find a shop with a piston grinder and get a set of 0.040'' ground down to size, and then see if you can find a set of 0.030 rings to gap to suit.
This will probably cost you more than finding another set of barrels and starting again with the 0.020 pistons
This will probably cost you more than finding another set of barrels and starting again with the 0.020 pistons
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Some people just chucked Cortina 1500 Pistons in .
As these require bigger holes , it lightens the Barrel .
As these require bigger holes , it lightens the Barrel .
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lcrken
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kommando and Matt,
those Cortina pistons are 81 mm bore and require sleeving the cylinders to install, as well as honing the rod small end out for the larger pin. I've done that, and it works fine, but it's not quite what Nigel is looking for.
Ken
those Cortina pistons are 81 mm bore and require sleeving the cylinders to install, as well as honing the rod small end out for the larger pin. I've done that, and it works fine, but it's not quite what Nigel is looking for.
Ken
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