4" PVC slipped through 6" clay?

When installing a new sewer line at my house, the goal is a complete 4" sch40 PVC mainline connection from the house to the curb, except that, in one small section, in which a concrete staircase is over the pipe, they're considering slipping the 4" PVC through through the existing 6" clay pipe (assuming the current outside line is 6" clay.)
The line within my house is already only 4" line, and 4" PVC is what they normally install around here for new residential lines as permitted by the code. If so, then slipping the 4" Sch40 PVC through existing 6" clay pipe is being considered for the problematic section where the concrete steps are located. The end result would be a completely-PVC connection from my house to the curb, except that one section of the PVC pipe would happen to have the old 6" clay pipe around it.
Would this be considered improper or is this acceptable?
Jeff
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I see nothing wrong in slipping the 4" PVC through the 6" clay pipe as long as there is continuous fall on the pipe (since you're using it as the invert.) Hopefully there is nothing wrong with that section of pipe as it will save you time and money. Avoid any joints in this section (not knowing how long it is.) Just remember that the clay pipe you're looking at might very well be 4". The wall thickness of clay pipe is so great, that it may appear to be 6" - a very common error - even by some pro's in a hurry. For your sake (and the sake of not removing the concrete steps) I hope it is. :-)
Don't forget to install a cleanout plug at every 90 turn of direction and at least every 50'. Most people like to run 75' - 100' for their clean outs, but I've found that most places that sell cables to the homeowner come in 50' lengths. You might even consider a two-way cleanout about mid-line. Just a thought . . .
Jim

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<<Avoid any joints in this section (not knowing how long it is.) >>
Thanks again for the reply. Please clarify what you mean by avoiding any joints in this section. Do you mean not to do this if the clay pipe has joint(s) in that section, or do you mean to make sure the section PVC pipe inserted isn't two separate sections of PVC joined together?
Thanks.
Jeff
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No joints in the PVC pipe. If the section is over 20' long - you'll have no choice since you can purchase the pipe in 20' lengths.

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Are you sure about the 4 inch line (outside of the house) as I used to inspect main line sewer installations and all the house connections were 6 inches going to the curb ? It should be easy enough to check tho....go to the building dept. and / or plumbers for advise in your area. There may be building code requirements for this.
Personally, unless there is a compelling reason, I'd rather have 6 inch pipe outside the house. Inside homes tho, it's usually 4 inches.

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I see 6" being used here as well. that other old clay stuff is usually 5"
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