doubts about the quality of the engines manufactured by estes

There is tons of info. Just do a search for “Estes”

Like Bob said, it’s really just a run it and see kind of thing. As mentioned the parts were largely leftovers and used to assemble engines. So you may get a good one.

Before Estes I think they were quite strikt on the selection of parts, as the machining was not accurate alone but hey also had to match pistons and cylinders (by hand using air) in order to get the desired fit. This was likely not done during the Estes era and it is more of a hit and miss on the fit you happen get in a particular engine.

The engine should still run and perform well. While these have tolerances that were loosened from the heyday of Cox (1960's) where everything was hand-fitted, they were still manufactured in smaller runs (2000-5000) in the '97-'98 range.

The Sure Starts were a different animal since they made so many of them and literally assembled all of the parts they had on hand without consistent checks on quality control. A lot of the questionable ones were parts on hand from late '99 for the Star Wars project that failed miserably.

Matt

Almost all my reedies dating back to the 60s and 70s have a mildly tight piston and cylinder fit, assuring lasting and excellent  compression even when hot, through many many hours of dependable use.
My NIB, paper boxed COX engines like BW and Texaco dating back to the 80s and 90s are already way sloppier with the piston falling out of the cylinder. This lose fit ensures an easy and good run but not that longevity what older production engines promise. Maybe the later made TeeDees from the 80s and 90s are still well fit but none have tapered bore.like those made till the early 70s.
I still love them all but must accept that cost saving efforts diluted the genuine quality of the engines from heydays.
Sic transit gloria mundi.