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Speed,Combat,Scale,Racing => Speed Talk => Topic started by: Dennis Toth on August 13, 2021, 11:57:28 AM
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While installing an ABC piston/liner set in a case recently I noticed that the top 3/8" of the liner fit very tight. I decided not to force it and see what others think about how it should fit.
My thinking is it should be a slip fit that requires just a slight push to seat it without heat. My concern about it fitting to tight is that the brass liner expands a bit faster than the Aluminum case and would start to distort and bind the piston at running temperature. What has been your experience?
Best, DennisT
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I know Brian Gardner says NOT to heat the case with his sets. But I haven't had one that didn't fit so I don't know the fix.
Craig
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.... the brass liner expands a bit faster than the Aluminum case and would.....
I don't think so.
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Craig,
Thanks for the input, that's what I was thinking would be the proper fit to just slip in.
Bill,
If you think about the ABC set up, the reason for the pinch near top dead center is so that as the engine heats up the brass will expand more than the high silicon aluminum piston and reach a running fit. With the brass liner in an aluminum case, if the fit is very tight it restricts the brass expansion and can cause binding with the piston. The fact that the case has cooling airflow makes thing worst.
For AAC I think it might be a little less of an issue, but the case will have air cooling it and expand a little less so might also need just a slip fit here also.
Best, DennisT
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Craig,
Thanks for the input, that's what I was thinking would be the proper fit to just slip in.
Bill,
If you think about the ABC set up, the reason for the pinch near top dead center is so that as the engine heats up the brass will expand more than the high silicon aluminum piston and reach a running fit. With the brass liner in an aluminum case, if the fit is very tight it restricts the brass expansion and can cause binding with the piston. The fact that the case has cooling airflow makes thing worst.
For AAC I think it might be a little less of an issue, but the case will have air cooling it and expand a little less so might also need just a slip fit here also.
Best, DennisT
When was the last time you saw an engine with the case cast in "high silicon" aluminum like the piston? Piston: yes; case: no.
Bottom line: the fit of the piston/liner/case is a combination of factors, one of which is the differential expansion rates of the materials involved.
Coupled with a lot of imagination on the part of the person deciding what it should be. :-)
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Bill makes a good point, only case I know of that is high SI Al is the K&B Sportster series, the current one offered by MECOA is the 28 and it is AAC. I did get curious and google expansion coefficients and found this site: https://www.sciencedirect.com/topics/engineering/linear-coefficient-of-expansion . OK for normal AL its coeff from table 21.1 is 25.5; Brass is 19 they don't have high si AL. so the AL case should expand a bit faster then the Brass liner, but it is cooled so who knows. I think I still like the slip fit as recommended by Brain Gardner.
Interesting the iron/ steel numbers are very close around 10.2/10.5 pretty good match for piston/cylinder.
Best, DennisT
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Bill makes a good point, only case I know of that is high SI Al is the K&B Sportster series, the current one offered by MECOA is the 28 and it is AAC. I did get curious and google expansion coefficients and found this site: https://www.sciencedirect.com/topics/engineering/linear-coefficient-of-expansion . OK for normal AL its coeff from table 21.1 is 25.5; Brass is 19 they don't have high si AL. so the AL case should expand a bit faster then the Brass liner, but it is cooled so who knows. I think I still like the slip fit as recommended by Brain Gardner.
Interesting the iron/ steel numbers are very close around 10.2/10.5 pretty good match for piston/cylinder.
Best, DennisT
Does the K&B Sporster series even have drop-in cylinder liners? For some reason I thought they were a chromed aluminum piston running directly on the high silicon case casting.
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From what I can tell the K&B is a cast case no liner. Not sure if the chrome is on the cylinder or piston.
Best, DennisT
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I dont think you mentioned what kind of case you are dealing with. Might help for an answer.
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Aluminum FOX 35 case. Factory ABC P/L set.
Best, DennisT
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OK, I may get pounded for this but I think you can lightly 'hone' the case with some 600 on your finger till it goes in with a moderate push. Also, have you tried a steel sleeve in there and also tried the brass in one or several other cases?
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Yes, steel sleeve fit fine in this case. When I get a chance I will mic the O.D. of the sleeve and see if it is larger at the top then bottom (pretty good change it is). I was thinking along the lines of the case honing rather then trying to reduce the sleeve, have more control.
Best, DennisT
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Are you sure it is the top of the liner that is the problem? After all, this is a fox you are talking about. I was wondering if the bottom of the case bore is a bit tight or dirty.
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OK, I may get pounded for this but I think you can lightly 'hone' the case with some 600 on your finger till it goes in with a moderate push. Also, have you tried a steel sleeve in there and also tried the brass in one or several other cases?
Carl, i was told by one of the best engine men in the country that a little sandpaper never hurt anything