I have a minor dilemma on my pistons.
I am running 74.5 (.60 over) listens and I can get rinds for those bit they are cast top rings.
I can get chrome but those are 75mm and I will need to file them down.
Note this the the race motor in my.Datsun that sees 8200 RPM.
So what would you choose?
File to fit. That way you can ensure the gap is exactly what you need. I wouldn't trust an unverified gap.
How are you planning to get 75mm rings to seal in your 74.5mm bore?
Fresh bore, or just a hone?
If just honed, the recommendation used to be cast, but I'm not sure if that's changed.
The larger rings are not going to have the correct radius for the smaller bore. They'll both have excessive drag and likely seal poorly at the same time. That's if they don't break when you compress them enough to fit in the bore.
You might be able to find chrome rings in the size you want by spec rather than application.
APEowner said:The larger rings are not going to have the correct radius for the smaller bore. They'll both have excessive drag and likely seal poorly at the same time. That's if they don't break when you compress them enough to fit in the bore.
You might be able to find chrome rings in the size you want by spec rather than application.
This was my concern as well. While the cast rings are not ideal they fit the bore.
I've filled rings for two strokes before but only because nothing else was available.
Buy a cheap hand-powered ring grinder and after that filing rings isn't hard, merely tedious. Embrace it and make sure it's right.
Make sure you debur the filed ends too.
Outside of the bore size issue, I was taught that chrome rings aren't used commonly anymore, because in theory they last longer, but in practice it isn't worth the trade off, the trade off being they are harder to seat and not tolerant at all of dirty oil or air.
Tom1200 said:APEowner said:The larger rings are not going to have the correct radius for the smaller bore. They'll both have excessive drag and likely seal poorly at the same time. That's if they don't break when you compress them enough to fit in the bore.
You might be able to find chrome rings in the size you want by spec rather than application.
This was my concern as well. While the cast rings are not ideal they fit the bore.
I've filled rings for two strokes before but only because nothing else was available.
What pistons are you using? There may be a better ring set that fits.
In reply to APEowner :
The pistons are some brand I got off Ebay. I bought them because they are flat tops i.e. higher compression.
I've used cheap brand pistons for years without issue.
The real solution is to simply buy forged pistons; I may do that for the next motor but didn't want the expense this time. Note I used forged pistons in the A15 motor because you couldn't get flat top pistons for that.
I'm surprised at the amount of misinformation mixed into this thread.
First off, whether or not the rings get filed, gaps can and must be checked.
The comment about 75mm rings being too big for the bore is just plain garbage. They would need ~1/16" filed off the end, and that's fine. If they were an inch too big, different story, but we're talking .020 in bore size here.
There have only ever been two reasons to run chrome rings. One is in Porsche motors with Alusil or Nikasil bore treatments (both very hard). The other is for off-road motors that see a lot of airborne particulates (dust). In both cases, the goal is to keep the top rings from wearing out quickly by using a very hard chrome plating on the face of the ring. The rings are not made of chrome, they're still iron, and have no performance benefit besides the face hardness. Chrome rings do take a long time to seat, and may never seat with improper breakin. Also require different bore finish.
I'd just plan on the plain rings and check gaps. If the last set didn't cause a problem, that says they're fine.
Nockenwelle said:I'm surprised at the amount of misinformation mixed into this thread.
First off, whether or not the rings get filed, gaps can and must be checked.
The comment about 75mm rings being too big for the bore is just plain garbage. They would need ~1/16" filed off the end, and that's fine. If they were an inch too big, different story, but we're talking .020 in bore size here.
There have only ever been two reasons to run chrome rings. One is in Porsche motors with Alusil or Nikasil bore treatments (both very hard). The other is for off-road motors that see a lot of airborne particulates (dust). In both cases, the goal is to keep the top rings from wearing out quickly by using a very hard chrome plating on the face of the ring. The rings are not made of chrome, they're still iron, and have no performance benefit besides the face hardness. Chrome rings do take a long time to seat, and may never seat with improper breakin. Also require different bore finish.
I'd just plan on the plain rings and check gaps. If the last set didn't cause a problem, that says they're fine.
The problem with running rings that are 0.020" oversize for the bore isn't the end gap. As you and others have pointed out it's pretty trivial to set the end gap. The problem is that rings are manufactured so that they're round and have a specific tension when installed in a specific bore size. When installed in a different bore size they're going to be less round and the tension will be effected. Twenty plus years ago some ring manufactures specified a tolerance of up to + 0.010" for a given ring size but I've never seen a minus tolerance. Here's a chart from a 2009 Hastings catalog.
HOW TO SELECT CORRECT OVERSIZE RINGS
Hastings Piston Rings are available in Std. size and the following oversizes:
.020", .030", .040", .060", .080", .100", .120" where required.
Contact Hastings Piston Ring Engineering Department at factory for non-cataloged ring applications, or unusual
oversizes. For sets in stock the following chart can be used.
The chart below gives the proper oversize rings to install for various oversize cylinders.
BORE DIAMETER USE RING SIZE
Std. to .010" ................................................STD
.020" to .029" .............................................. .020"
.030" to .039" .............................................. .030"
.040" to .049" .............................................. .040"
.060" to .069" .............................................. .060"
.080" to .089" .............................................. .080"
.100" to .109" .............................................. .100"
.120" to .129" .............................................. .120"
So, what happens if you use an oversize ring in a smaller bore? The ring tension will be higher than spec and they won't seal correctly. How bad will it be? I don't really know. I'm sure the engine would run and maybe it would be OK for a street motor where you had no other choice but it's clearly not the correct way to do it and since we're talking about a race motor why would you do that?
As to ring types, you're correct. We're talking coatings not ring material. A plain cast ring is not a bad choice in this application and I should have said that in my earlier posts. My excuse is that I was typing on my phone and I was going to clarify my recommendation if or when Tom came back with some piston specs.
I typically use moly coated rings in vintage race motors when they're legal but it depends on the exact application including what part of the country their running in. Here in the southwest where we're often racing in very dusty conditions and chrome rings may be appropriate.
In reply to APEowner :
The 74.5mm pistons have 2mm first and second groove and a 4mm oil groove.
It took 4 months for a set to pop up. Toyota 4K engines use the same rings. Mitsubishi 4G36 is the same as well but finding those is even harder.
Most rings available for that bore size are to thin; motorcycles use 1mm. Honda & Toyota engines are typically 1.5mm thick.
Tom1200 said:In reply to APEowner :
The 74.5mm pistons have 2mm first and second groove and a 4mm oil groove.
It took 4 months for a set to pop up. Toyota 4K engines use the same rings. Mitsubishi 4G36 is the same as well but finding those is even harder.
Most rings available for that bore size are to thin; motorcycles use 1mm. Honda & Toyota engines are typically 1.5mm thick.
Hastings makes a ring for that, but it's for .060 over.
6714s060.
Looked again, think it's for a .060 over 73mm bore, which would be 74.5
Tom1200 said:In reply to APEowner :
The 74.5mm pistons have 2mm first and second groove and a 4mm oil groove.
It took 4 months for a set to pop up. Toyota 4K engines use the same rings. Mitsubishi 4G36 is the same as well but finding those is even harder.
Most rings available for that bore size are to thin; motorcycles use 1mm. Honda & Toyota engines are typically 1.5mm thick.
Yeah, that's an unusual configuration. I'm not seeing anything in the catalogs I have. My next step would be to reach out to Total Seal and see if they have a standard part that would fit. They can make anything you want but for your application if you're doing custom it would make more sense to spring for custom pistons with a more standard ring size.
I'm happy to reach out to Total Seal for you but if you want to just do it yourself here's the link Total Seal - Find a ring. They'll need the grove depths in addition to the information you've provided already.
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