View Full Version : Bang for buck L20b build - L28 flat top piston to valve clearances

14-10-11, 12:03 PM
In the midst of piecing together a bang for buck L20b for my bluebird and have a few queries regarding valve to piston clearances running L28 flat top pistons.

Some quick specifications of what im looking at running with.

Head: U67 (open chamber) L20B – 42mm intake / 35mm exhaust (although looking to upgrade to 44mm L28 intake).
Cam: Tighe 72 works (profile 359 - http://www.tighecams.com.au/nissan-menu.htm)
Block: / crank / rods: L20B
Head gasket thickness: 1.2mm
Pistons: late L28 ‘flat tops’

Head has never been machined so is stock thickness (~ 108mm) and have CC’d the combustion chamber to just over 45CC which run through the Ozdat ‘engine design utility’ throws out a static compression of 10.03 with L28 flat tops.

That said I’m planning on relieving some of the shrouds within the combustion chamber so above figure may come back to 9.8 if I remove ~1CC of material.

Basically I am unfamiliar with L28 pistons (cheapo replacement ACL variety), are these flycut OEM? – am worried about valve to piston clearance with flat tops – especially with 72 works cam.

On a side note I considered a Z22 and Z24 stokers (have cranks available) but wanting to keep costs down will probably stick with stock crank & overbore to around 2.05L / 100cc increase in capacity – I’ve decided on not changing induction so limiting fun factor will be 40mm mikuni/solex carbs.

(Unless there is some magic combination of wrecker parts that can be assembled cheaply with minimal machining that is – chasing bang for buck over dollars on this build)

Thanks in advance.

14-10-11, 12:37 PM
I recently rebuilt my L18 (yeah, I know, its not an L20B, but close enough) using L28 flat tops (yes they are flat, no valve reliefs), 44mm inlets and a 72 deg cam.

I ended up with less than 1mm clearance, wasn't happy with that so I decided to flycut the pistons a little. If you're really going "bang for buck", I can post a few pics of how I did it using a hand drill, an old valve and a few other odds and ends.

Its going to be close enough that if I were you I'd mock it up and see exactly what clearance you get. They are old motors, unless you've owned it since new you don't know how far the valve seats are recessed through wear / being recut / etc, don't know exactly if / how much has been taken off the block / head in its lifetime, etc.

Dummy assemble the engine, put a bit of plasticine on top of the piston, then turn it over carefully by hand (and stop if it feels like valves and pistons have come together). Measure how thin the plasticine is compressed to. Flycut if you're not happy with the clearance.


14-10-11, 12:49 PM
Thanks for you reply Dave, this is exactly the information i was after.... God bless PF :D

While I haven’t owned the motor since new was pulled out of a bluebird with 90,000km since new (one owner type car - block and cylinder head is mint!) - so fingers crossed everything comes up within spec… but always the unknown.

If you don’t mind, I have a few questions on your build.

What head did you slip onto that combo?

Overbore to 86mm or did you go further (40thou / 87mm)?

Would be great if you were to post pics of your hand relief (LOL) or any other pics of your build you may have.

Re – valve to piston clearance, i guess i could also run with a slightly thicker 1.5mm gasket if required (which will drop compression to 9.8 - which would still be plenty acceptable).

Motherfucker Jones
14-10-11, 01:24 PM
The other thing you can do to for minor valve to piston adjustments is advance/retard cam timing. 2 degrees can make all the difference sometimes. I vaguely remember there's some tricky way to adjust cam timing on L series with stock cam gears. Can't remember it though…

I'd look at .060" (1.5mm) as minimum v to p. Exhaust clearance is more critical because the piston chases it up the bore as it's closing. If it floats and hangs open it'll get hit. All depends on how hard you want to rev it too. L series I'm guessing lots :)

14-10-11, 01:56 PM
I used an A87 head, went for 86.5mm pistons (I planned to go to 87mm, but couldn't get the pistons at the time and needed it back together quickly for a race meeting).

I'll post some pics of my special tool / build later tonight when I'm home.

You can adjust cam timing using the three standard holes in an L-series cam sprocket, but my experience is that they are relatively sensitive to changes in cam timing. Personally, I'd do it right rather than work around the problem.

On my engine, exhaust valve clearance wasn't as close as inlet (due to the valve angle on an L-series compared to the piston, and the fact that exhausts are smaller)

14-10-11, 02:55 PM
Cheers mate.

A87 head, some nice comp there with flat tops.

Any other tips and tricks (cheap worthwhile) chucking a L series together?

Am quite tempted using one of these Z22/24 cranks i have available - but reckon it'll end up costing a truckload for little gain (limiting factor being 40mm carbs and starting to get away from being a 'budget' build).

If it somehow helps - this is the current state of play.


Series II electronic ignition & small runner intake manifold.... undecided on keeping the stock fuel pump, like the idea / look of it.

14-10-11, 04:20 PM
With the 40 mm carbs have a look at the venturi sizes.
I assume that they are removable like the Weber/Dellorto.

When I played with 1750/2000 Alfa engines donkey's years ago the carbies, usually 40 Dellortos with a 32 mm venturi, were a limiting factor. Boring them to 35 or 36 mm and reshaping was a very helpful and cheap upgrade.

14-10-11, 07:51 PM
Go Z18 brah

14-10-11, 07:56 PM
There are two sizes of the factory L series mechanical fuel pumps, can't remember off the top of my head which motor's had the bigger one though.
I ran the larger of the mechanical pumps on my old L20 with 2 x 45DCOE's and that made 94rwkw so the pumps flow plenty.

14-10-11, 07:57 PM
^ x 2

14-10-11, 07:59 PM
Jon, my motor has flat tops and an a87. .... if you want a cheap build you should be able to pick up a motor for nothing with an a87 head (I got a l20 with u67 for free, and an l18 with a87 for free too)

14-10-11, 10:32 PM
bang for buck: find L6 flywheel (240\260 possibly 280) because they are lighter than the L4 item. Add Patrol clutch and have fun

14-10-11, 10:43 PM
I didn't do anything real "trick" with my L18 - as far as headwork goes, it had already been ported a little,and I did some more. I completely removed the "bosses" around the valve guides, then shortened the valve guides a little to reduce obstructions in the port. 44mm inlets, 35mm exhausts. Flat top L28 pistons, stock rods were cleaned up, polished and balanced (overall, plus balanced end-for-end) on my $35 eBay scales. Pistons were also balanced.

All the "soft" oil gallery plugs were removed, 40 years of crap cleaned out, then tapped the ends and put tapered plugs in instead. Same for the crank, then had it balanced. Main and big end bearing clearances were set up a little "loose" as per the old factory rally-prep guide (I think there's a copy on Datsport's website ?). Rocker arm pads were ground just a touch (on a belt sander) so the new cam would bed in nicely. Cam was set up by colouring in the rocker arm pads with texta and mucking round with different sized bits of feeler gauge between the end of the rocker and the lash pad till the texta was wiped off the centre of the rocker pad to determine the correct size lash pad, then buying that size from Stewart Wilkins.

I still run SU carbs (but 1 3/4" genuine british ones) on a modified, ported stock Datsun SU manifold. Plus a bluebird electronic dizzy. All good for 102 rwhp - not a huge amount, but the whole rebuild owes me no more than $1000.

To flycut the pistons, I took two old valves (one exhaust, one inlet), brazed a little bit of tool steel onto each of them, then ground them into cutters that extended just past the edge of the valve. Insert the pistons one at a time (set up on the conrod using a gudgeon pin that I turned down a whisker for 2/3 of its length so I didn't have to use a press to get the pistons on and off). Put the two cutting tools (old valves) in an old head I had lying round, mocked it up on the engine and turned it to TDC.

I made an adjustable "stop" out of an old bolt head - then I lowered the cutter so it was just touching the top of the piston (had to rotate it and used a dial gauge to check when it was at its "highest" setting). Then I just stuck a feeler of the appropriate thickness to get the required valve-to-piston clearance between the top of the valve guide and the bottom of the "stopper" tool, tightened up the pinch bolt in the stopper, and removed the dial gauge. Put the end of the valve into a drill chuck, turn on the drill, and slowly and very carefully cut the valve relief into the top of the piston (aided by lots of lubricant on the piston top and plenty of finger-crossing). It did the job with only a little "chattering" if I wasn't careful or tried to cut too much in one go. I did finish them off with wet & dry to ensure there were no rough spots.

Once each piston was done, I removed the head, moved onto the next piston and set everything up again. Its survived quite a few race meetings now, and is stll running nicely. One thing you could do differently is determine exactly where the valve and piston are closest - I suspect its probably not exactly at TDC (would depend on the cam profil) but using TDC worked for me (plus I made the flycut a little larger diameter than the valves).

Setup to find "highest" setting of cutter, ie if you rotate the valve so the valve edge rather than the cutter point is touching the piston top, the dial gauge reads lower.


Closeup of the home-made depth-of-cut stopper - use a feeler gauge to set the depthe of cut by raising the "stopper" that distance above the top of the valve guide.


The finished article


Mucking round with bits of feeler guage to determine las pad thickness required (the little bits under the end of the rocker arm, not the one between cam ad rocker)


Set it up so the wipe pattern looks like this


And not this


14-10-11, 10:48 PM
Oh, I still use the stock fuel pump. It is perfectly adequate and very not-complex (no electricity needed, etc).

14-10-11, 10:57 PM
My L20B Rally engine was bored to 89mm with pistons 10tho proud of the bore and used a Works round exhaust port head with a very small chamber and a 76 works cam, I had 13-1 comp (Racefuel) and had to cut very deep valve reliefs. make sure you use an L 24-26-28 harmonic balancer or better still a Stewart wilkins BMW based Balancer. you need to use 6cyl waterpump and alt bracket with the 6cyl balancer.
I ran 45mm carbs with 40mm chokes but 50s would have been much better. This motor is still around and id love to buy it back

14-10-11, 11:03 PM
Oh, I still use the stock fuel pump. It is perfectly adequate and very not-complex (no electricity needed, etc).

Early to mid 80's might have even gone a bit later mazda 929's the LTD version ran a electric fuel pump but still had a carby, shit easy to mount as it was/is external. Unsure of the flow figures might be worth the time to research it

14-10-11, 11:25 PM
bluebird s2 runs simple electric fuel pump

14-10-11, 11:25 PM
Any truth in the story that the twinport exhaust manifolds are better then extractors for a street L18/L20B? When I was researching stuff for the 720 the exhaust bloke next too work reckoned my stock manifold with twin pipes to the back of the cab, Kingcab, then into a single pipe would be much better then extractors for the street. He was running the exact setup on his L20B powered 620, twin Dellortos and nice cam. Although his dumped in front of the rear wheel, not an option here for obvious reasons.

14-10-11, 11:29 PM
it'll last foirever being a cast iron piece and they look like they flow well enough. kinda reminds me of that krusty burger joke

14-10-11, 11:53 PM
Thanks for all your replies so far guys (especially dave1600... champion) - really great info here.

To add a little more to the story, i actually have a A87 head - picked one up a couple a months ago for what i thought was a reasonable price.

A87 head, larger valves and porting


All was great till i cracked out the vernier calipers - came up at 105mm (with around 107 being acceptable & 106.5 considered usable)

so after a machine to slap onto my L20B we are talking in the high 104mm thickness which im lead to believe will cause all sorts of chain/tensioner issues - so gave up on that idea.

Unless you blokes reckon it can be salvaged?

In comparison the 90,000km U67 head (still fairly oily from CC'ing) - open chamber


15-10-11, 12:21 AM
I believe you can make up spacers for the pedastools to correct chain tension issues and run thicker lash pads to recify reduced cam contact

15-10-11, 12:25 AM
I believe you can make up spacers for the pedastools to correct chain tension issues and run thicker lash pads to recify reduced cam contact

40 thou max from my limited net research for the cam towers spacers which still leaves me around 40 thou short and one hell of a lash pad (with my tighe cam).

im all ears if it cam be made to work though.

15-10-11, 12:28 AM
I'm no TK, he's like 19 letters better than me (AK)

15-10-11, 12:41 AM
not that I'll think it will help on the spacer info but I have a 73' printing of the "How to modify DATSUN 510 610 240Z engines and chassis" book. Apparently it has some info that was left out in later printings, what exactly I don't know but it's been a pretty good buy so far!

Motherfucker Jones
15-10-11, 12:43 PM
The other thing is how much comp will you have with the A87 head? Especially if it's been cut that much

15-10-11, 02:40 PM
lots ... lots of comp rocks. just ask my datto

15-10-11, 04:12 PM
The other thing is how much comp will you have with the A87 head? Especially if it's been cut that much

absolutely, I cc'd the head at 34cc so combined with stock L20b 11.34cc dish pistons it came in at around 9.8, or probably bang on 10.0 with another light skim.

That was the plan anyways.

But as far as i can see the head is unusable at that sort of thickness?

16-10-11, 02:10 PM
Had a bit of a play around with my A87 head and tensioner today, i think i may actually be able to use the head after all.

chucked the stock U67 head back on (unmachined 108.2mm thick) and measured the 'slack' in the tensioner - came in at 7.5mm (older chain and guides probably not 100% setup).

then chucked on the decked A87 head (105mm thick) - 'slack' increased to 10.22mm

(both heads installed on stock 1.2mm gasket)

so ripped the tensioner off and decided the easiest way to fix this would be to dowel the stock holes and drill new ones ~3mm further back - pushing the tensioner out the required ~3mm (after checking the block oil port would still line up)





with the guides slightly modified tensioner 'slack' now sits at around 3mm (new chain and i'd expect that to be around ~1mm which is around what the factory manual quotes).

So unless any of you datto blokes reckon thats shithouse i'm probably going to call that issue sorted.

As for timing apparently each chain link is worth 18 deg - each link is 10mm.

so with the head being decked around 3.5mm the timing by my calculations - will be retarded by approx 6.3 deg.

A possible fix for this is the standard cam gear allows timing to be set at 0, +4 and +8 deg

so running with the +4 setting my cam timing will be retarded 2.3 deg or +8 setting cam timing will be set at approx 1.7 deg advanced

Any thoughts here guys? - any advice is greatly appreciated.

Motherfucker Jones
16-10-11, 03:11 PM
Only issue as far as the tensioner would be increased wear around the bottom edge as it's seeing more chain contact, but it looks workable.

As far as cam timing, I would get a degree wheel and dial it in properly. Too many variables in manufacture of gears and cams etc to guess it with maths. It's not difficult and then you know where things are setup for future builds etc (ie this cam works well 2 deg advanced) if you want to do the same combo again

16-10-11, 06:34 PM
In line with "bang for buck" - the guy who dyno'd my L18 a while back (before the rebuild) wasn't happy with the cam timing on mine.

He removed the dowel that holds the cam sprocket onto the cam, the set the cam up with a dial gauge to where he wanted it. Then stuffed rags down the timing cover and simply drilled a hole straight through the cam sprocket and into the end of the cam. Whack a dowel into the new hole, bolt it up and all good.....

As shocked as I was, it performed much better after this and ran for another few years before needing a full rebuild.

I'm a little undecided on the tensioner "fix". I can't see why it won't work, but I'm a little sceptical (although I'm not sure why).

One solution would be the Kameari timing setup which replaces the standard slack-side guide and tensioner with another idler sprocket, but they are rather exxy.

16-10-11, 09:08 PM
Don't nismo have a racing tensioner where you manually wind out how much you want?

17-10-11, 07:15 AM
If it wasn't a bang for buck job, that deck height would be fixable - I've seen it done before but I don't imagine it is cheap to build up the entire surface then machine it and clean it all up again.

17-10-11, 09:17 AM
fireyone - i cant find it, only other solution i can find is the Kameari timing gear setup that Dave1600 suggested.... Going to order a new chain today and have a bit of a play.

Shifty - the current thickness (if workable) is perfect for use on the pistons my engine is fitted with, should come in a shade over 10:1

Next project is to find cam lashpads (SW motorsports / datsport?) and pickup a degree wheel too, and work out how to use it :D

17-10-11, 09:33 AM
I got my lashpads from SW, got a degree wheel from a random website - print it out, stick it on a bit of cardboard.

You'll also have to factor in that the valves will now be a few millimetres closer to the piston crown with the head machined that much.

EDIT: just realised you're using dished pistons with that head - depending on how much dish the pistons have (compared to flattops), may not cause too much drama.

17-10-11, 09:44 AM
cheers mate, pistons have epic dish, 11cc - so should be all sorted in that area.

(but the right size to work with a 34cc combustion chamber for a street type setup)

17-10-11, 09:51 AM
p.s. all this talk of flat tops and not one single dynosteve comment (oxy, dave or myshorty mustn't read down this far :lol:)

17-10-11, 10:04 AM
BTW I got a Nismo cam sprocket that's surplus to my requirements. I think it's Nismo anyway, it's like a stock one but has lots of peg holes. One peg hole has been drilled out by some goose in Japan, but as long as that isn't the one you need...

17-10-11, 10:16 AM
All this talk of building L series makes me want to go to the shed and build one!

17-10-11, 10:21 AM
Sounds like exactly what im after Babalouie, have seen pics of them, should have 8 holes, 2 deg adjustment between 0 & 18 deg (one chain link).

This is my understanding anyways.

Shoot me a PM with what your chasing for it... cheers!!

17-10-11, 10:32 AM
Sounds like exactly what im after Babalouie, have seen pics of them, should have 8 holes, 2 deg adjustment between 0 & 18 deg (one chain link).

This is my understanding anyways.

Shoot me a PM with what your chasing for it... cheers!!
Nah, it's yours, just PM me with your addy and sort me out for postage later.

17-10-11, 10:42 AM
wow absolute champion!!!.... well looks like timing issues are sorted :D

PM on its way - thanks again!

17-10-11, 10:44 AM
.... well looks like timing issues are sorted :D
...as long as the peghole you want to use isn't the drilled-out one :lol:

Motherfucker Jones
17-10-11, 12:46 PM
and pickup a degree wheel too, and work out how to use it :D

Theyre not that scary. Just make sure you work out where TDC is accurately or everything else is a waste of time. I make an 'adjustable' pointer out of a piece of wire and use a dial indicator on top of the piston. With the dial indicator you'll notice the piston is at TDC for a few degrees of crank rotation so you can't just put it at the top and call it spot on. Get your pointer roughly on TDC then I usually go .100"(any distance works) either side of TDC. Check how many degrees it reads each way with the piston down .100". You then need to 'adjust' the pointer until it reads the same degrees in each direction at .100"

Once you have this equal your degree wheel will show TDC exactly. Alternatively if you have the head on, you can use a piston stop. Rotate the engine around until it touches the stop, check how many degrees you have on the wheel then rotate the opposite direction until you touch the stop. Same deal just get the degree readings the same in each direction, take the stop out and you have TDC. Best to do this with the cam off so valves don't open and fuck shit up or pick a fight with the piston stop.

You can easily make a piston stop by gutting an old spark plug, tap the inside and put a bolt through the middle. If you want to be nice to your pistons round the end of the bolt

Once you have your cam and shit bolted on you can degree your cam. Most cam companies will give you a lift at TDC and this is probably the easiest method to set it up.

Hope some of that makes sense...

18-10-11, 04:22 PM
Without trying to unnecessarily bump this thread thanks for your reply Mrs Jones - extremely helpful.

And Babs - haha i hope so too!

edit - found this degree wheel too.


19-10-11, 07:28 PM
A or L series threads can handle a bump, not like those hydrogen cell threads that could blow up if you bump them...

19-10-11, 07:49 PM
I just got to drive my Rally spec L18 with twin 45's for the first time in 5 weeks ..... fuck its fun

Motherfucker Jones
19-10-11, 07:57 PM
For some reason I really like L series engines. I reckon they look cool with some weebers hangin off em and the bottom end is strong as fuck. They're just a nice simple engine for a pushrod retard like me

19-10-11, 08:42 PM
I like both, just got my Y running on monday after a few month stint of sitting around after engine mount broke and discovered the carby was in bad shape. Goes like the clappers now with the welded up manifold and 38 DGES