U20 too much overbored???

[Labubre]

Hi,

Bought a 2000 roadster recently.
Seller stated a rebuild engine 10 years ago.

I just discovered that during rebuild, block has been overbored to fit Dome Pistons 0.06.

Isn't that too much for a U20?
Any experience/opinion to share about that move?

Is there a specific head gasket for that in order to stand compression rate and also the new cylinder diamater?
Thanks

Seb

[Bwk2000]

Might want to send Dean (Datsunparts.comInc) a PM. Since he sells the full engine rebuild kits using the 0.06” domed pistons for both the 1600 & 2000 (link below), he can probably hook you up with the gaskets you need.

https://datsunparts.com/products/datsun ... s?variant=

[JT68]

.060 is not too big. Nor does it require a special gasket.

However domed 1600 pistons will certainly make the compression ration quite high. Probably around 11:1 depending on the head chamber size.
A CR that high is a poor choice for a roadster street engine.

[Labubre]

Any risk of regular head gasket issues?
I saw on sellers maintenance invoices that he used to order a U20 thick head gasket to Dean (taht i did contacted by the way)
If compression rate too high, how to make it more convenient for a street roadster?

[Gregs672000]

.060 is not too big. Nor does it require a special gasket.

However domed 1600 pistons will certainly make the compression ration quite high. Probably around 11:1 depending on the head chamber size.
A CR that high is a poor choice for a roadster street engine.

As one who has run 11.7 to 1 and now closer to 11 to 1 I concur that you will likely have issues if you plan to run 92 pump fuel. Options would be to replace the pistons; have the current ones machined to reduce the compression; purchase a 123 distributor to create a timing map that will reduce the chance for knock at specific rpms (though true detonation is not spark induced); or run a REAL octane booster like Boostane (good stuff and fairly economical). Consider posting pics of your pistons so we can see if anything has been done to reduce compression already, as I recall an R16 piston increased compression to somewhere around 12 to 12.5 to 1 as done in the Bob Sharp U20 race engines (see tech wiki) without a reduction in the dome (?). As JT said, it will depend on the head chamber so post pics of that too. There are methods and formulas to calculate compression ratio but there's a specific process... I believe mine was done using clay to represent the space and a beaker of water to measure displacement but I didn't do it... JT or other builders could probably say more on that, or YouTube (?).

This also becomes a bit more problematic depending upon what induction system you have... SUs vs Mikuni/Weber/Dellorto type carbs, as the most likely moment it will ping/knock is on sudden acceleration where an injection of fuel from an accelerator pump to quickly and thoroughly richen the mix (as with a Mikuni type) is not something you get with an SU. I've been down many of these roads, and my engine is now fuel injected and crank-fired with a custom timing map in order to run 92 octane fuel, and on hotter days (85 and up) I put in a splash of Boostane just to add a bit more protection. All of this can be done, however IMHO the best advice is to try and calculate your ratio and/or machine or replace your pistons if it's over 10.75 (max) to 1 unless you want to open up a can of worms. If you're OK having to add an octane booster then consider Boostane, as you will be able to get 92 high enough to work no problem and it doesn't take much. I would run a 123 distributor so you can modify the timing where you need to... ignition timing is where power is found, but with higher compression some time on a dyno to create a timing map is very helpful (I will be scheduling time to finalize mine this spring and I'd be happy to share what MY engine wants... may give some idea though there are many variables).

Hope this helps...

[JT68]

Any risk of regular head gasket issues?
I saw on sellers maintenance invoices that he used to order a U20 thick head gasket to Dean (taht i did contacted by the way)
If compression rate too high, how to make it more convenient for a street roadster?

The Nissan gasket is excellent, but with R16 pistons and a even slightly shaved U20 head, there is a high likelihood of piston-to-head contact.
**and** If you clearance the head to fit, as Greg mentioned, you'll be pushing 12:1. That isn't going to work out well for a street engine.

The best correction is eliminate the domes/replace the pistons, then you have your choice of head gaskets.

You could also use a thick head gasket, but only if the head has been cut by about .060" already. --If the head has not been cut "enough", and you try to use a very thick gasket, you won't have enough upper chain length to fit and your cam timing would be quite wrong as well.

[redroadster]

I've dome pistons in the SPL about 175 psi
Use 91 or better , adjust to find the best ign timing , has good accel my motorcycle is 12.5 to 1 , I even run it on mid grade occasionally , np
.060 i...s 30 thousands a side

[JT68]

yes, but we are talking about u20 here. With the additional stroke and even a moderately cut head, it's CR is way too high for street use. Would be retarded.

[Datsunscott]

A splash of low lead avgas can make a big difference- and smells good to boot.
It has 5 times what the car had in tetraethyl lead, So to add some really helps this gasahol junk we are getting.
A trip to the airport to get 5 gallons gas to test your airplane motor lol will go a long way.

PS if you are on the edge of detonation might need to richen up the mixture to bring the true air fuel ration back to original burn.
10 percent alcohol burns a little different and leaner. On a vintage ford falcon i drilled the main jet only 1 jet drill size up and it helped.
I'm not sure if you need to polish the needs down a but or if you can raise them or lower the jet? I'm still learning.

[Gregs672000]

Anyone else notice that our new friend is in FRANCE! 311s is world-wide!
Also, we're (or at least I have) been assuming that the "domed" piston is an R16 piston, but it could be an Arias or similar... images would be helpful!

[JT68]

A "splash" of AVgas will not do much of anything. A full tank is a different story and 8$ a gallon or so here locally. For 12:1 you are smack dab in race gas territory or 12$+/gallon. Does that really make sense for a street car? no.

Greg is correct, Hi CR makes no sense on a street roadster unless you have very sophisticated engine management and the ability to program it. (an accurate knock sensor and EFI??) or you don't mind paying for 100octane fuel?

If you have to retard the timing and waste more fuel sending it out the tailpipe, all you are doing is reducing the thermal efficiency of the engine and reducing actual power output. Correctly tuned engines run MUCH better. 11 or 12:1 is great for a track car, but that is about it.

How much is 100 octane in France?

[Labubre]

reference 1189E: datsun roadster dome pistons - 0.06" as mentionned on invoice
Purchased back in 2021

On seller site this reference still exists with another description: earch on seller site shows "Datsun Roadster 1600 Mild Dome Pistons SETS w/Rings"
In my case rings were purchased aside and where identified "1600 Hastings 0.06" ring set"

For fule and Octane, lowest we have in France is 95. I always use the most expensive 98

[Labubre]

digging in seller documentations, found that
2013: 1st head gasket during engine's rebuild
2016: same thick head gasket purchase
2019: again
So should have been changed in 2022 but decision to sell
Of course, i was not informed about that by seller

So wrong configuration here:
- piston & compression rate
- head gasket quality (same reference since 2013)
- wrong head block surfacing

Hoping that block is not dead (as cannot be bored again), i will do the safest configuration for a "street" roadster to put her back on our roads
Will follow your reco!

[redroadster]

Yes ..but what is the compression going down the road ? , what's left of raw gas to cyl ?
and the piston moving faster than the combustion? This is part of a 4-5 day engine perf.class

[JT68]

Pretty sure Europe measures octane differently than the US:

"Thus, 97 octane “super unleaded” in Britain is roughly equivalent to 91 octane premium in the United States"

"Datsun Roadster 1600 Mild Dome Pistons SETS" are 1600 domed pistons-not a good choice in a u20.