we like to have the pistons when boring,we`ve seen on more than one occasion the same brand pistons mic out different,ALWAYS check "pre-bored" cylinders

 

Ok, I understand the need to supply the pistons when getting your jugs bored. What about HD new pistons with HD new jugs, surely you must improve your odds of a good fit?

Jim

 

seen probs with factory 1550 HD cylinders as well as 883-1200 factory cylinders,when we bore them,we KNOW their right on

 

And then there were the HD cast flatops in the std box piston marked std and rings were std but what the heck not one but two sets in a row of these about a year back that were actually .005 OS
And then there was the 113" 4.060 barrels the customer bought and gave me .005 OS CP pistons, but in the plates it would have taken .007 to clean them due to a distorted factory bore job and then the pistons would have had .0045 clearance when they called for .0025.
Give the shop the pistons!!!
Send them in a USPS flat rate box for 10 bucks priority mail. Done!

 

So, what everyone is saying is the big bore kits that the dealers are installing are a crap shoot as far as consistency is concerned? I don't find it hard to believe with mass produced pieces. With all the kits being installed I rarely, if ever, hear about any problems with them though. So, is it really that bad? Or is it just a situation where someone can do better, or sleep better at night, knowing the cylinders were bored to a certain spec?

I guess my question is about tolerances. If I pick up a 22851-99a flat top kit and a used set of cylinders what piston to wall clearance should the machine shop be shooting for? I've read quite a few differing opinions and numbers. What would a shade tree mechanic tell a machine to do when boring out the cylinders? Thanks

 

It can be that bad, read Deweysheads post again. Just not a good idea to take a set of pistons/cylinders out of the box an install them without checking fitment. That would be like buying new pair of Luchesse boots, size 10.5D without trying them on because you have always worn Justin Ropers in size 10.5D and they fit fine. What's the rush, measure the piston to bore fitment!

Piston to bore tolearances are available in service manuals and on line. Typically, the cast piston will be fitted with .001"-.0015" clearance and forged pistons will be fitted with .002"-.004" clearance; these will vary with the manufacturer and/or the builders experience. A shade tree mechanic, if he thought about what he was doing, would by .010" OS pistons, knowing that matching a set of new standard size pistons with a used set of cylinders was a crap shoot. Then he would instruct the machinist to verify that the cylinder bore is true and within spec and, once verified, the machinist would then bore/hone the cylinders to fit the new OS pistons.

 

I think it's the old "measure twice, cut once" adage.

 

The spec sheet that comes with the pistons says "Fit in cylinder .0006 - .0017". These are different than what you posted and a bit confusing. Are you saying you would run a cast piston with .015" clearance (referencing your post above)? That seems quite a bit beyond the FSM wear limit of .005". What am I missing?

 

You are not missing anything and I am not suggesting .015" clearance for a cast piston. I dropped a zero; should have read .001"-.0015", which most that read the post probably understood that the .015" was a typo. However, thanks for pointing it out; I have edited my original post to reflect accurate information.

The fitment specs you refer to are for cast pistons. As you can see, the margin for error in the fitment specs is about .001" which is why bore dimensions to determine if the bore is true and piston dimensions must be measured with accurate instruments, torque plates must be used and the machinist must know what he is doing.

 

This is another VERY interesting and helpful thread as I am about ready (like in the next few days) to install the HD 95" cast pistons and cylinders I bought from Chicago HD a few months back. When I read the 1st few posts yesterday I got to thinking that I should have the sizes verified beforehand, so I called 3 different local dealers to find out if they check cyl., rings and pistons from the Moco before they install them in their rebuild work. They told me they don't bother with that and just slap 'em in. I'm like yikes! So I called the Moco to find out if they verify specs before shipping a "matched" set of pistons and cylinders. Of course I knew the answer before I asked the question. After reading the latest posts today, I called a 4th dealer who's about an hour from me and they said that yes, they do check cylinders (on torque plates) and pistons beforehand and are glad they do because they have gotten out-of-spec cylinders and pistons from the Moco, but since they caught the f.u. beforehand, didn't install the parts. They could check mine for a 1 hour service fee of $70.00 and give me written documentation on the measurements. Seems like good insurance for peace of mind. Now I just have to figure out if I trust 'em to do the job properly before I install them. Thanks again guys for sharing your knowledge.