InfamousRS

I went to a local machine shoe today to find out abou over boring my block when I rebuild it.

I plan on using CTR pistons from http://www.jdmhondaparts.com. they say this about the piston "Your choice of Standard or .25 Oversized." Im gussing that the .25 means .25mm, right?

Anyways, I told the machine shop that my current bore is 81mm and that the pistons Im planning on buying are 81.25mm. He responded by saying that would not be near enough to bore out far enough for the new rings to seat properly. He said .25mm is only 10 thousandth of an inch.

What have you guys experienced with .25mm over. Why would honda make a piston that size if it would not even work?

hybridreams

Hold on...rephrase what the shop told you; I'm having trouble understanding. Is the shop saying the .25mm extra bore would not be enough to seat the piston rings properly?

InfamousRS

Yes, that is pretty much what he said. he said its not enough to take the imperfections out of the sleeve walls.

mingchi96

bullshit.. take it somewhere else. Most times, you dont even have to rebore the block to get the rings to seat, just a light rehone that you yourself can do with a tool that can be rented from autozone. If a machine shope told me that, I would leave rite away.

StinkySohc

i think ".25 Oversized" means 25cc over 1800cc's. The .25 they were talking about would be of a liter. something like that... still learnin.


[Modified by StinkySohc, 7:32 AM 4/20/2002]

InfamousRS

So how big is the piston then? if its increasing in displacement, the piston would have to be bigger then 81mm.

Oh, I don't think im going to let this guy touch my block. He doesn't seem to know to much. He asked me what I was going to be doing, and I told him that I was going to be using a Vtec head on my non-Vtec block. I also told him I was going to run 12.0:1 CR pistons. He said, somthing like, "you cant do that just because a magazine says you can." and that "not even the most extream race motors run that high of compression." Then he asked what kind of fuel I was going to run, I said 92 octain. And he giggled and said "not on pump gas."

I think I will be finding a deffernt machine shop.

JD_B18CDX

Machine shops do not usually know much about VTEC frank engines.
They are usually very conservative poeple but know a lot about tolerances. I would understand why they would react the way they did when you mentioned a 12:1 compression ratio. They are most likey used to old v8 engines when 8.8:1 was considered high.
I myself sometimes cannot believe that my motorcycle engine 11.5:1 and is still carburated.

If a mahinist tells you that .25 mm is not enough material to remove imperfections, chances are he is correct. Assuming the machine shop you went to is not a hole in the wall kinda business.
A boring and honing machine are a precision tool and very expensive.

A few years back when I was rebuilding a Mitsu engine, I got pretty much the same response. I need a bigger oversize piston because my cylinders are scuffed up. The guy I was talking took some time to explain the process of boring and honing which was very much appreciated.
Basicaly there could be several reasons why a machinist would ask for a bigger oversize piston. 2 main reasons are.
1. Your cylinders have scratches
2. Your cylinders are not straight... because of uneven wear(U cannot see this on by eye but a good measuring tool will)

We chatted about this a few posts back. As I have said, a good machinist will tell you what pistons you need. Sure you can force the issue but I would rather trust a person who have been doing this kinda things for years and and rely on his business for a living.

If you are not comfortable with the guy, get a second opinion. It does not cost much. Just your a little bit more time.

You are in one of the critical phase of engine rebuilding. Your decisions here would affect the performance and durability of your project.

Hope this helps out.