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 Piston ring gap
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Betsy67
V4 Fanatic

United Kingdom
308 Posts

Posted - 07 Oct 2018 :  06:34:05 Show Profile Reply with Quote
Im just starting to trial build my 1.7 engine and got a bit puzzled by the ring gap.
I have a spare stripped engine (1500) and some HC 1.7 pistons. The bores in the block didnt seem worn and had no scoring or ridges. Ive measured them best I can (cheap Chinese verniers which were discarded from wifeys marketing department - for good reason!!) and would appear to be 90mm.
As everything is on a budget I want to recycle wherever possible, so onto the ring gap. I have fitted the top compression ring from a 1.5 piston and one from a 1.7 piston in all 4 bores. The variation is between 1.2 and 1.3mm in which ever combination of ring /bore you choose. However in my manual the gap is recommended (working from memory ) between .250 and .4mm. I find it a bit strange that rings from two separate pistons/engines have worn ?the same.
Any thoughts ?

beta
V4 Fanatic

Ireland
277 Posts

Posted - 09 Oct 2018 :  12:09:19 Show Profile Reply with Quote
I think I have set of new over-sized rings for a 1700 - this would allow you to carefully file down the new rings to the gap you need.
I can check later of you wish.
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Betsy67
V4 Fanatic

United Kingdom
308 Posts

Posted - 09 Oct 2018 :  18:27:11 Show Profile Reply with Quote
Not too sure on this but dont oversize rings require oversized pistons too ?
Im sort of guessing that 1500 and 1700 rings are the same ? With the the bore being the same size etc.
If anyone ( Melle ? ) has any thoughts whether your offer is workable well move things along.
Thank you.
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Derek
V4 Guru

United Kingdom
1816 Posts

Posted - 10 Oct 2018 :  10:55:29 Show Profile Reply with Quote
How are you measuring the ring gap?

"When measuring piston ring end gap, check the gap with the rings at the top and the bottom of the bore. If the bore has taper wear (bores typically wear most at the top), the end gap will be larger at the top and smaller at the bottom of the bore. Use the bottom position to set the end gap." (MiniMania)

Your two sets of numbers may be correct based on the above.
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Betsy67
V4 Fanatic

United Kingdom
308 Posts

Posted - 10 Oct 2018 :  18:44:56 Show Profile Reply with Quote
I measured at the top, so will have a check at the bottom too. The bores werent very polished and didnt take too long to hone, so presumed they werent too worn. Worth trying at the bottom for peace of mind - a job for the weekend.

Edited by - Betsy67 on 10 Oct 2018 18:46:52
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melle
V4 Guru

United Kingdom
2563 Posts

Posted - 11 Oct 2018 :  09:59:37 Show Profile Visit melle's Homepage Reply with Quote
It's been a good few years since I last measured piston ring gaps, but 1.2-1.3 mm sounds a lot to me. When building up an engine with used parts from different origins I think I would fit new rings anyway to be honest. 1.5 and 1.7 rings are identical.

In my opinion vernier calipers ("guessometers") are not the right tool to accurately measure bores. I use telescoping gauges in combination with external micrometers. Once you've learned how to use them they can give you very accurate results. "Proper" three point cylinder bore gauges are very, very expensive.

www.saabv4.com
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RhysN
V4 Fan

United Kingdom
179 Posts

Posted - 11 Oct 2018 :  11:53:12 Show Profile Send RhysN an AOL message Reply with Quote
This is the accepted formula, sorry it's in inches, but....
.004 inches times the bore diameter for the top piston compression ring. .005 for the second compression ring.
Doing the maths for a 90 mm bore;
that works to 0.01417". That translates to 0.3599 mm.
Please check the maths!
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melle
V4 Guru

United Kingdom
2563 Posts

Posted - 11 Oct 2018 :  12:11:06 Show Profile Visit melle's Homepage Reply with Quote
Specs can be found in the factory manual or Haynes.

www.saabv4.com
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Betsy67
V4 Fanatic

United Kingdom
308 Posts

Posted - 11 Oct 2018 :  19:13:50 Show Profile Reply with Quote
Unfortunately Ive lost my access to proper measuring equipment as Ive just changed jobs. Before I go any further More measurements are needed.
I quoted the manual tolerances in my first post, so the 0.35 mm looks right.
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beta
V4 Fanatic

Ireland
277 Posts

Posted - 12 Oct 2018 :  10:38:46 Show Profile Reply with Quote
I checked, the new piston ring set that I have are 90.8mm +.5mm so are the second oversize for a 90.8mm bore. They are new in the box.
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Betsy67
V4 Fanatic

United Kingdom
308 Posts

Posted - 15 Oct 2018 :  20:00:03 Show Profile Reply with Quote
Had a bit of time over the weekend, mainly due to the bad weather !
Measured a couple of different rings in a couple of bores and yes there is a small variation - probably about 0.3 mm. I borrowed a decent set of verniers (all I can get my hands on) and all are showing under 90mm with very little variation between. Think Ill order a set of new rings and take it from there.
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