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No Compression After Poncam Install

robbo_rb180
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robbo_rb180 Grand Master

robbo_rb180

Posted
Posted

i have put a set of poncams in my rb25 and sent the car away to get it wired up.

when my mechanic went to turn it over it just kept winding over. after a quick compression check No.1 cyclinder has no compression.

we have checked the timing, its all good.

what would cause this?

its a s2 rb25det with tomei "series 2" cams (late r33 with r34 CAS).

my mechanic thinks i have the wrong cams but i checked them and they are the right cams.

any help would be appreciated.

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https://www.sau.com.au/forums/topic/329039-no-compression-after-poncam-install/
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Deano 1 Mentor

Deano 1

Posted
Posted

are you sure u got all the right clearance's between the cam lob and the lil cup thing that sits on the vavles? it must have the the vavles always open just a lil bit so theres no comp.

my 2 cents

snozzle Mentor

snozzle

Posted
Posted (edited)

if you were working on the engine while number 1 cyl 1 was at tdc which is wat u should be doing and moved the cam substantially without a timing belt on you may have opened your valves and smacked the piston causing them to bend, quite possible that ur mechanic tried to remove (undo) the cam gear after he took the timing belt off and the cam rotated during this without the crank moving at the same time

Edited by snozzle
snozzle Mentor

snozzle

Posted
Posted
are you sure u got all the right clearance's between the cam lob and the lil cup thing that sits on the vavles? it must have the the vavles always open just a lil bit so theres no comp.

my 2 cents

his engine has hydraulic lifters, no clearance adjustment is possible, neither is it needed

robbo_rb180 Grand Master

robbo_rb180

Posted
  • Author
Posted
if you were working on the engine while number 1 cyl 1 was at tdc which is wat u should be doing and moved the cam substantially without a timing belt on you may have opened your valves and smacked the piston causing them to bend, quite possible that ur mechanic tried to remove (undo) the cam gear after he took the timing belt off and the cam rotated during this without the crank moving at the same time

i did the cams myself when the motor was out. i made sure i didn't move anything after i set no.1 @ TDC. mechanic only just took the timing cover off to check the timing. when i get the car back home i will pull the cam covers off & see whats happened.

i don't think its a broken cam as everything rotates smoothly.

hope it hasn't bent valve(s). :(

i have a reco'd head & headgasket if it has though.

if anyone has any other ideas what it might be post it up.

cheers

rob

robbo_rb180 Grand Master

robbo_rb180

Posted
  • Author
Posted
deano must have thought it was an RB26.

do any of the other cylinders have compression?

i wish it was a 26, solid lifters FTW.

i haven't checked them yet, it will be the 1st thing i do when i get the car home.

DVS JEZ Grand Master

DVS JEZ

Posted
Posted

hope u havent killed the valves in cyl 1 and cyl 6

i did it on my 1st rb26 with poncams i bent cyl 6 valves and i did it by the book.

a mate told me after that to get piston 1 on TDC as normal but then turn it back until all the pistons are at an even height, then install cams (no way the valves can hit cause the pistons are down,) get the cams to the right position then turn the crank forward till at TDC then install the belt, has worked every time for me now. annoying way to learn it but

PM-R33 Grand Master

PM-R33

Posted
Posted (edited)

Hmmm I swear I did my camshafts a few times with the engine at TDC and never bent anything :P

Doesn't the engine manual say to make sure cylinder one is at TDC?

Edited by PM-R33
boostn0199 Grand Master

boostn0199

Posted
Posted

I think the manual says at TDC but cant remember exactly. But yeah if it is at tdc provided the cam is in the exact correct position and tightened it shouldnt bend the valves I wouldnt think.

jiffo Mentor

jiffo

Posted
Posted
Hmmm I swear I did my camshafts a few times with the engine at TDC and never bent anything :(

Doesn't the engine manual say to make sure cylinder one is at TDC?

Probably does say TDC. It's a dreadful manual and shows no understanding of what MIGHT happen.

Compression TDC on No1 means No6 inlet and exhaust valves are in the process of changing over, opening/closing.

Only takes a slight cam rotation and the valves will hit the piston.

Anyone who has installed cams knows they like to rotate slightly as the caps are wound down, so it's simply good practice to rotate the crank slightly off TDC so there is no possibility of any valves interfering.

It also gives you the chance to test rotate your cams (without hitting pistons) to ensure everything is happy.

Oddly enough, Nissan chose No1 BDC for its RD28 camshaft install, cambelt timing the works. A much better option as at No1 BDC, the only protruding valves are poking into cylinders with the pistons way down.

post-73571-1279253698_thumb.jpg

snozzle Mentor

snozzle

Posted
Posted

Ive never had a problem working at TDC, on the rb engine you have to move the inlet cam about 30 degrees anti clock wise to even begin to open, thats quite alot of movenent.. i think you just have to be abit more careful when your bolting teh cam shaft down, make sure the lobes on cyl 1 are looking 10 o clock on inlet side and 2 o clock on the exh

everyone has their own methods, i dont see why you HAVE to work at TDC but its just what i have always done

PM-R33 Grand Master

PM-R33

Posted
Posted (edited)

Yeah I have lost count the number of times I changed cams troubleshooting my stupid problems (Stock cams, HKS cams, Tomei cams) and I think I worked at TDC every time, but I always made sure the cams didn't rotate much when bolting the cam caps down.

Edited by PM-R33
Adriano Grand Master

Adriano

Posted
Posted
Ive never had a problem working at TDC, on the rb engine you have to move the inlet cam about 30 degrees anti clock wise to even begin to open, thats quite alot of movenent.. i think you just have to be abit more careful when your bolting teh cam shaft down, make sure the lobes on cyl 1 are looking 10 o clock on inlet side and 2 o clock on the exh

everyone has their own methods, i dont see why you HAVE to work at TDC but its just what i have always done

Even though you have to rotate the cam 30 degrees for the valve to start opening, #6 piston is also at tdc, and both valves are open.

DVS JEZ Grand Master

DVS JEZ

Posted
Posted

I built my rb26 then after a couple months and bit by the power bug i put in poncams and had it at tdc cause thats how u line the timing belt up. I bent the valves in cyl 6. Poncams have more lift then stock cams so its even easier to push the valves into the piston

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