Can I get +025 Pistons for an 850?

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Nigeldtr

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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?
 
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,
 
Nigeldtr said:
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?
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Ouch, it seems to happen so frequently, the usual cast of characters, good machine work is hard to find. I believe .020, .040, .060" is the progression of oversizes for these.
 
+.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.
 
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.
 
Postby Bruce MacGregor » Wed Jun 26, 2013 9:33 am
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,
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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.

Can I get +025 Pistons for an 850?


Can I get +025 Pistons for an 850?


Can I get +025 Pistons for an 850?
 
Nigeldtr said:
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?
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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.
 
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!
 
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.
 
Bruce MacGregor said:
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!
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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.

Whether a Norton or a Lawn Boy, shit should be right.
 
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: :D
 
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.

Due to the danger of pistons cracking most shops just won't do it, that being said it is very easy to do and all you need are a lathe and a coarse knurling tool to do it (I usually grip them by the ring land and rotate the piston against the tool). What you need to find is an old timer not afraid of doing things the old fashioned way and understands what it means to just get it running...the rebuild can come later.

Generally when you knurl you go oversize and then polish with emery back to size, knurling to an accurate diameter is next to impossible. Usually you need the block to polish the pistons to fit after.

You might want to try some of the more remote communities near you that cater to the local farmers, more than a few tractors finished harvest with a knurled piston or two in her innards!


Hope you get some luck finding a machine shop with a machinist who went to the school of hard knocks, they are getting harder to find nowadays.
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Here is another option, Put teflon buttons in the skirts of the pistons. I did this many years ago,about 73 , and put them in a C/gas vega and ran low 10's with it. used Dykes type rings and never did have a problem with it
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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
 
Some people just chucked Cortina 1500 Pistons in .
As these require bigger holes , it lightens the Barrel .
 
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
 
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